Indispensable Co-litigation in Japan (Koyū Hitsuyōteki Kyōdō Soshō) - Part 1: What Distinguishes it from Similar Co-litigation and How Do Courts Approach "Uniformity of Judgment"?

In the realm of Japanese civil procedure, most lawsuits involve a single plaintiff against a single defendant, or multiple parties who join voluntarily for convenience (ordinary co-litigation). However, certain types of disputes necessitate the participation of multiple specific individuals as either plaintiffs or defendants for the lawsuit to proceed validly and for its judgment to be effective. This is known as "necessary co-litigation" (hitsuyōteki kyōdō soshō - 必要的共同訴訟).

Japanese law further distinguishes between two primary forms of necessary co-litigation:

1. Indispensable Necessary Co-litigation (Koyū Hitsuyōteki Kyōdō Soshō - 固有必要的共同訴訟)
2. Similar Necessary Co-litigation (Ruiji Hitsuyōteki Kyōdō Soshō - 類似必要的共同訴訟)

While both aim for a consistent outcome among the involved parties, the reasons for this necessity and the procedural implications differ significantly. This article, the first of a two-part series on indispensable co-litigation, will focus on defining what makes a co-litigation "indispensable," how it differs from its "similar" counterpart, and the critical concept of "Uniformity of Judgment" (gōitsu kakutei - 合一確定) that underpins these procedural constructs.

The Core Distinction: "Necessity of Joint Litigation" vs. Extension of Judgment Effects

The traditional distinction between these two forms of necessary co-litigation hinges on two key elements: the "necessity of joint litigation" from the outset, and the source of the requirement for a "uniformity of judgment."

Indispensable Necessary Co-litigation (Koyū Hitsuyōteki Kyōdō Soshō)

This form is characterized by two fundamental requirements:

1. Necessity of Joint Litigation (soshō kyōdō no hitsuyō - 訴訟共同の必要): For the lawsuit to be properly constituted and for the court to render a valid judgment on the merits, all indispensable parties must be joined in the litigation, either as co-plaintiffs or as co-defendants. The absence of even one indispensable party typically means that the joined parties lack proper standing to sue or be sued with respect to the entire subject matter, potentially leading to a dismissal of the action on procedural grounds.
2. Uniformity of Judgment (gōitsu kakutei - 合一確定): The judgment rendered in the case must be legally and factually consistent for all indispensable co-litigants. They must, in essence, all win or all lose together on the common issue that binds them. To facilitate this, Article 40 of the Japanese Code of Civil Procedure (CCP) imposes special procedural rules. These include provisions that an advantageous procedural act by one co-litigant benefits all, while a disadvantageous act (like an admission of a crucial fact) by one might not bind the others if they contest it. Furthermore, the proceedings for all indispensable co-litigants are typically unified, meaning they are handled together, with unified evidence and a single, consistent judgment.

Similar Necessary Co-litigation (Ruiji Hitsuyōteki Kyōdō Soshō)

This form is primarily characterized by the Uniformity of Judgment requirement, but the reason for this uniformity differs from indispensable co-litigation.

• No Initial Necessity of Joint Litigation: Critically, there is no initial procedural mandate that all similarly situated persons must sue or be sued together. Each individual who falls into this category can typically initiate or be subject to a lawsuit independently regarding their own rights or obligations.
• Uniformity via Extension of Judgment Effects: The requirement for uniformity arises because the effects of a judgment (判決効 - hanketsu-kō) rendered for or against one such person are legally stipulated (by statute or established case law) to extend to other similarly situated individuals, even if those others were not formal parties to the suit. Examples include certain shareholder actions where a judgment might bind all shareholders, or actions to annul a corporate resolution where the outcome affects all members.
• Consequence if Multiple Parties Do Join: If several such individuals do choose to join as co-litigants in a single action, then Article 40 CCP applies to them as well. This is not because they had to sue together initially, but because, once joined, the judgment for one will invariably affect the others due to the pre-existing rule of judgment effect extension. Therefore, to avoid internal contradictions within that single proceeding (where the judgment for co-litigant A would extend to co-litigant B, and vice-versa), their outcomes within that joint suit must be uniform.
• Intervention: A third party to whom the judgment effects would extend can often join the pending litigation through a mechanism known as "intervention by an indispensable co-litigant" (kyōdō soshō sanka - 共同訴訟参加) under Article 52 CCP, effectively becoming a party to ensure their interests are represented.

Thus, the fundamental distinction lies in the source of the necessity: in indispensable co-litigation, it's often the very nature of the underlying substantive right or legal relationship that demands joint participation for a valid adjudication. In similar necessary co-litigation, it's primarily the pre-ordained broader reach of the judgment's effects that necessitates uniformity if multiple affected persons are party to the same suit.

Traditional Basis for Indispensable Necessary Co-litigation: Joint Management and Disposition Rights

Historically, the principal ground for classifying a lawsuit as requiring indispensable necessary co-litigation was rooted in substantive law: specifically, when the management and disposition rights (kanri shobun-ken - 管理処分権) over the subject matter of the dispute were legally vested in multiple persons jointly, such that they could only exercise these rights collectively, not individually. This is the requirement of "joint exercise" (kyōdō kōshi no hitsuyōsei - 共同行使の必要性).

Examples include:

• Property held under certain forms of co-ownership:
  • Collective Ownership (sōyū - 総有): A form of ownership by an unincorporated association where individual members have rights of use and benefit but no individual shares, and the property is managed and disposed of by the association as a whole.
  • Joint Ownership (gōyū - 合有): A form of ownership where multiple persons own an undivided interest in property, typically with restrictions on individual disposition and often with rights of survivorship (though Japanese case law tends not to distinctly recognize gōyū as separate from ordinary co-ownership, often treating it under the rules of kyōyū).
  • For Ordinary Co-ownership (kyōyū - 共有), where co-owners have distinct, alienable shares (akin to tenancy in common): Indispensable co-litigation is less common. It is typically required only when the "co-ownership right itself" is being asserted or defended against a third party (e.g., an action by all co-owners to confirm their collective title against an external claimant, as in Supreme Court, October 7, 1971, Minshu Vol. 25, No. 7, p. 885). Actions concerning individual co-ownership shares or rights that can be exercised individually (such as "acts of preservation" under Article 252, proviso, of the Civil Code, or claims concerning indivisible obligations under Article 428 of the Civil Code) generally do not require indispensable co-litigation.
• Formative Actions (keisei no uttae - 形成の訴え) Necessarily Affecting Multiple Parties: These are actions that seek to create, alter, or extinguish a legal relationship that inherently involves multiple specific individuals in an indivisible way. For example:
  • An action by a third party to annul a marriage must typically name both spouses as defendants.
  • An action to dismiss a company director often requires both the director and the company to be parties.
    The rationale here is often described as ensuring "substantive procedural guarantee" (jisshitsu-teki tetsuzuki hoshō - 実質的手続保障) for all individuals whose core legal status or relationship is directly and indivisibly targeted by the requested judgment.
• Procedurally Mandated Joinder: Situations where procedural law itself dictates joint action, such as suits involving multiple bankruptcy trustees acting jointly for the bankruptcy estate, or suits brought by or against multiple "selected parties" (sentei tōjisha - 選定当事者) who are appointed to represent the interests of a larger group. Here, the right to conduct the litigation itself is effectively co-owned or must be exercised jointly.

Expanding Horizons: Indispensable Co-litigation Based on Procedural Necessity and Effective Dispute Resolution

While the traditional basis for indispensable co-litigation was heavily tied to substantive law requirements for joint action, Japanese case law has, particularly in recent decades, recognized indispensable co-litigation in other contexts where overriding procedural considerations and the practical need for effective and unified dispute resolution are paramount. This expansion often occurs in areas where a single, consistent judgment binding all key stakeholders is deemed essential for the stability of subsequent legal processes or for the definitive settlement of a multi-faceted dispute.

A. Boundary Determination Suits (Kyōkai Kakutei Soshō - 境界確定訴訟) Involving Co-owned Land

A boundary determination suit in Japan is unique; it's considered a "formal formative action" (keishiki-teki keisei soshō - 形式的形成訴訟). The court does not merely declare a pre-existing boundary based on rights but rather authoritatively demarcates or establishes the boundary line, which is often referred to as the "public law boundary" (kōhō-jō no hikkai - 公法上の筆界) between parcels of land.

The Supreme Court, in a decision on December 9, 1971 (Minshu Vol. 25, No. 9, p. 1457), held that when either or both of the adjacent lands involved in such a suit are co-owned, the boundary determination suit is indispensable necessary co-litigation. This requires all co-owners of the land on one side and all co-owners of the land on the other side to be parties to the action.

Rationale: Although individual co-owners may not have private management or disposition rights over the abstract "public law boundary" itself, its determination directly and indivisibly affects the scope, extent, and value of their respective co-owned properties. To achieve a single, authoritative, and uniformly binding determination of this boundary, and thereby effectively resolve the dispute for all concerned, the participation of all those with the "closest legal interest"—the co-owners of the abutting parcels—is deemed indispensable. This classification is driven more by the procedural need for a comprehensive and final resolution concerning the boundary than by a substantive law rule dictating joint exercise of a private right over the boundary line itself.

B. Actions to Confirm Property as Part of a Deceased's Estate (Isan Kakunin no Uttae - 遺産確認の訴え)

Disputes over the assets comprising a decedent's estate are common. An "action to confirm property as part of an estate" seeks a judicial declaration that specific assets belong to the deceased's estate and are, therefore, subject to co-ownership by all the legal heirs (isan kyōyū - 遺産共有) before any formal division of the estate occurs.

Historically, a related type of action—an action to declare a will invalid—has generally not been treated by Japanese courts as indispensable necessary co-litigation (e.g., Supreme Court, September 11, 1981, Minshu Vol. 35, No. 6, p. 1013). This could sometimes lead to fragmented or inconsistent outcomes among different heirs concerning the will's validity and, consequently, the composition of the estate.

The isan kakunin no uttae emerged as a means to address this. The Supreme Court first affirmed the admissibility of such actions (March 13, 1986, Minshu Vol. 40, No. 2, p. 389), recognizing their utility. A judgment in such an action would have res judicata effect on the issue of whether the specified property is part of the estate, which is a crucial preliminary question for any subsequent estate division proceedings (遺産分割 - isan bunkatsu), whether by agreement among heirs or by court order.

Subsequently, in a landmark decision on March 28, 1989 (Minshu Vol. 43, No. 3, p. 167), the Supreme Court held that this isan kakunin no uttae is indeed indispensable necessary co-litigation, requiring all co-heirs to be parties to the suit.

Primary Rationale: While the substantive co-ownership of an undivided estate by heirs (isan kyōyū) is a factor, the compelling reason for this classification is largely procedural. Estate division proceedings inherently require the participation of all co-heirs for a valid and binding division. Therefore, any preliminary lawsuit that aims to definitively establish the corpus of that estate (i.e., what assets are subject to division) must also involve all co-heirs to ensure its judgment is uniformly binding and thus effective as a foundation for the subsequent estate division. The "necessity of ensuring the effectiveness and consistency of the subsequent estate division proceedings" drives the requirement for joint litigation in the confirmation suit.

C. Actions for Confirmation of Non-Existence of Heirship (Sōzokunin no Chii Fusonzai Kakunin no Uttae - 相続人の地位不存在確認の訴え)

Complementary to determining what assets are in an estate is determining who are the legal heirs entitled to share in it. An "action for confirmation of non-existence of heirship status" seeks a judicial declaration that a particular individual is not a legal heir of the deceased.

The Supreme Court (e.g., July 6, 2004, Minshu Vol. 58, No. 5, p. 1319; and March 16, 2010, Minshu Vol. 64, No. 2, p. 498) has classified these actions also as indispensable necessary co-litigation among all potential co-heirs.

Rationale: The reasoning mirrors that for the isan kakunin no uttae. The determination of who is and who is not an heir is fundamental to defining the parties who must participate in any subsequent estate division. To ensure a single, uniformly binding judgment that definitively settles the scope of rightful heirs for the purpose of a conclusive estate division, all potential co-heirs must be parties to the action confirming (or denying) a particular individual's heirship status.

The Meaning of "Uniformity of Judgment" (Gōitsu Kakutei) in These Contexts

The imperative for "uniformity of judgment" in indispensable necessary co-litigation means that the court's decision on the common issue that necessitates the joint litigation must be consistent for all the indispensable parties.

• In the traditional types rooted in joint substantive rights (e.g., collective ownership claims), uniformity means that all co-litigants who must act jointly to assert or defend the right achieve the same ultimate outcome on that right (e.g., their collective claim is either upheld for all or dismissed for all).
• In the more procedurally driven types (such as the estate-related confirmation suits or boundary determinations):
  • Uniformity means that the determination on the specific common issue (e.g., "Is this asset part of the estate?", "Is this person an heir?", "Where is the boundary located?") is legally binding and consistent for all the indispensable parties.
  • This ensures that subsequent legal processes (like estate division or land registration based on the boundary) can proceed from a clear, undisputed, and uniformly established legal premise concerning these foundational matters.
  • Achieving this requires all indispensable parties to be formally part of the litigation so that the judgment's res judicata can bind them all equally with respect to the determined issue. This is distinct from "Similar Necessary Co-litigation," where uniformity for non-parties is often achieved through the statutory extension of judgment effects, not necessarily by compelling their initial joinder as a prerequisite for the suit's validity.

Conclusion (Part 1)

Indispensable Necessary Co-litigation is a vital procedural construct in Japan designed to ensure that certain types of legal disputes, due to their inherent nature or their impact on subsequent legal processes, are resolved comprehensively and consistently for all essential parties. While traditionally anchored in substantive law principles requiring the joint exercise of rights (especially concerning management and disposition of property), Japanese case law has demonstrably broadened the application of this doctrine. Courts now also designate certain actions as indispensable necessary co-litigation based on overriding procedural necessities, particularly the need for a single, uniformly binding judgment that can serve as a stable foundation for subsequent related legal proceedings, such as estate divisions or the definitive settlement of land boundaries.

The defining features remain twofold: first, all parties deemed indispensable must participate in the lawsuit for it to proceed validly (the "necessity of joint litigation"). Second, the judgment rendered on the common issue that binds these parties must be uniform for all of them (the "uniformity of judgment"), a consistency facilitated by special procedural rules such as those found in Article 40 of the CCP. The next part of this series will explore the significant practical challenges that arise when some of these indispensable parties are unwilling to voluntarily join the lawsuit as co-plaintiffs.