"Naien" or De Facto Marriage in Japan: What Legal Protections are Afforded to Unmarried Couples?

Japanese Attorney at Law - Bengoshi L.L.

• 10 min read

In Japan, while formal marriage registration (kon'in todoke) is the sole pathway to acquiring the full legal status of a married couple, a distinct and legally significant category exists for couples who cohabit and share a life akin to marriage without such registration. This relationship is known as naien (内縁), often translated as "de facto marriage" or "common-law marriage," though it differs substantially from the common-law marriage recognized in some U.S. states. Over decades, Japanese courts and specific legislative acts have developed a framework of protections for individuals in naien relationships, recognizing their social reality while maintaining a distinction from formally registered marriages. This article explores the legal status and protections afforded to these unmarried cohabiting couples in Japan.

Historically, the Japanese legal system was stringent: no registration meant no marital rights. However, social realities, where couples lived as spouses without formalizing their union for various reasons (including past institutional impediments under the pre-WWII "Ie" system or simple personal choice), led to situations of hardship, particularly for women, upon the breakdown of such relationships or the death of a partner. This spurred a gradual judicial evolution in the treatment of naien.

A. From Non-Recognition to Quasi-Marital Status

Initially, courts were reluctant to accord any legal significance to naien. The first step towards protection came through the application of contract law, specifically the concept of a "promise to marry" (婚姻予約 - kon'in yoyaku). In a landmark decision by the Great Court of Cassation (Japan's highest court before the Supreme Court) on January 26, 1915 (大(連)判大正4年1月26日民録21巻49頁), the court recognized that the unjust termination of a relationship that had the characteristics of a marriage, even without registration, could be seen as a breach of a promise to formalize the union, giving rise to claims for damages.

However, academics and later courts found the "promise to marry" theory insufficient to capture the reality of naien relationships, where parties often considered themselves already married in substance, not merely intending to marry in the future. This led to the development and eventual adoption of the "quasi-marriage" (準婚関係 - jun-kon kankei) theory. This theory posits that a naien relationship, if it possesses the factual substance of a marriage (cohabitation with the intent to be spouses, mutual cooperation, and support), should be treated, by analogy, similarly to a legal marriage for many purposes, especially concerning the rights and obligations between the partners themselves.

Key judicial milestones in this evolution include:

  • The Great Court of Cassation decision of May 12, 1919 (大判大正8年5月12日民録25巻760頁), which allowed a naien husband to claim damages against a third party who had an affair with his naien wife.
  • The "Hanshin Dentetsu case" (大判昭和7年10月6日民集11巻2036頁), where a naien wife was recognized as having a claim for loss of support due to the wrongful death of her naien husband.
  • A post-WWII Supreme Court decision on April 11, 1958 (最判昭和33年4月11日民集12巻5号789頁), which firmly adopted the quasi-marriage theory in a case of unjust termination of a naien relationship. The Court allowed claims for solatium (isharyō) for emotional distress and applied marital expense sharing rules (Article 760 of the Civil Code) by analogy.

B. Rationale for Protection

The legal protection extended to naien relationships stems from several underlying rationales:

  • Preventing Unjust Outcomes: To protect the often more vulnerable party (historically, usually the woman) from being left with no recourse after investing years in a relationship that mirrored a marriage in all but formal registration.
  • Reflecting Social Reality: To acknowledge and give legal consequence to stable, marriage-like cohabitation that is widely understood by the parties and their community as a spousal relationship.
  • Equity and Fairness: To ensure a degree of fairness in the distribution of assets accumulated through joint efforts during the naien and to provide support where one party is left in need after its termination.

While not identical to legal marriage, naien relationships that meet the criteria of a de facto marital union receive significant legal protections in several key areas:

A. Dissolution of the Naien Relationship

The breakdown or termination of a naien relationship is treated by courts in a manner largely analogous to the dissolution of a legal marriage (divorce).

  1. Unjust Termination (Futō Haki - 内縁の不当破棄):
    If one partner unilaterally and without just cause terminates the naien relationship, the other partner can claim damages. This is akin to a fault-based divorce.
    • Solatium (Isharyō): Damages for emotional distress caused by the unjust termination are commonly awarded.
    • Property Division (Zaisan Bun'yo - 財産分与): The principles of property division applicable to divorced couples (Article 768 of the Civil Code) are applied by analogy. Assets accumulated through the couple's joint efforts during the naien are subject to division, taking into account each partner's contributions, including non-monetary ones like domestic labor. Such claims are typically handled by the Family Court through adjudication if agreement cannot be reached.
    • Third-Party Interference: If a third party (e.g., a parent of one of the partners) unjustifiably interferes and causes the breakdown of the naien, they may also be held liable in tort for damages. The Supreme Court affirmed this in a decision on February 1, 1963 (最判昭和38年2月1日民集17巻1号160頁), where a father-in-law's actions led to the dissolution of his son's naien relationship.

B. Rights Upon the Death of a Naien Partner

This is an area where the distinction between naien and legal marriage remains stark, particularly concerning inheritance.

  1. No Statutory Inheritance Rights: A surviving naien spouse has no automatic right to inherit from the deceased partner's estate under the intestacy rules of the Civil Code. Inheritance rights are reserved for legally recognized spouses and blood relatives.
  2. Property Claims based on Contribution or Agreement:
    • Co-ownership of Assets: If property was acquired during the naien through the joint funds or efforts of both partners, the surviving naien spouse may be able to claim a share based on principles of co-ownership, even if the property was registered in the deceased partner's sole name. For instance, the Osaka High Court decision of November 30, 1982 (大阪高判昭和57年11月30日家月36巻1号139頁) recognized a naien wife's co-ownership share in a business and related real estate due to her significant contributions as a joint manager.
    • No Post-Mortem Property Division Against Heirs: The Supreme Court, in a decision on March 10, 2000 (最判平成12年3月10日民集54巻3号1040頁), clarified that the divorce-related property division rules (Article 768) cannot be applied by analogy to allow a surviving naien spouse to claim a share of the deceased's estate from the legal heirs. The Court reasoned that this would improperly conflate property division principles with the distinct legal framework of inheritance.
  3. Residential Rights:
    • Rented Property: If the deceased naien partner was the sole lessee of their shared home, the surviving partner's right to continue residing there can be precarious. The Act on Land and Building Leases (借地借家法 - Shakuchi Shakka Hō, Article 36, formerly Article 7-2 of the old Tenant Protection Act) allows a cohabiting partner (including a naien spouse) to succeed to the leasehold rights if the lessee dies without legal heirs. However, if there are legal heirs, they inherit the lease. Courts have, in some cases, allowed the surviving naien spouse to assert the heir's leasehold rights against the landlord to continue residing (e.g., Supreme Court decision of February 21, 1967 (最判昭和42年2月21日民集21巻1号155頁)), but this protection is not absolute and depends on the circumstances, particularly the relationship with the heirs.
    • Property Owned by Deceased Partner: If the shared home was owned solely by the deceased partner, the surviving naien spouse generally has no right to continue living there against the wishes of the legal heirs. Limited protection has sometimes been granted under the "abuse of right" doctrine if eviction by heirs is deemed excessively harsh.
    • Co-owned Property: In a significant ruling concerning property co-owned by the naien couple and used as their residence or for a joint business, the Supreme Court on February 26, 1998 (最判平成10年2月26日民集52巻1号255頁), inferred an implicit agreement between the partners. This agreement was presumed to allow the survivor to continue using the property for the same purposes, free of charge, after the other's death until a formal division of the co-owned property. This prevents heirs from claiming rent for the deceased's share during this interim period.
  4. Social Security and Employment-Related Survivor Benefits:
    This is an area where naien spouses have gained considerable recognition. Many Japanese social security laws (e.g., for pensions, health insurance, workers' compensation) and internal regulations for employment-related death gratuities or survivor benefits now explicitly include or have been interpreted by courts to include naien spouses as eligible beneficiaries. Often, a naien spouse who was dependent on the deceased may be prioritized over a legal spouse if the legal marriage was defunct and the naien relationship was the substantive family unit. The Supreme Court decision of January 31, 1985 (最判昭和60年1月31日家月37巻8号39頁), concerning a private university's death gratuity, interpreted an ambiguous "survivor" (izoku) clause to include the naien wife, emphasizing the purpose of such benefits to support those who were factually dependent on the deceased.

C. Other Marital Effects Applied by Analogy During the Relationship

During the subsistence of the naien relationship, courts often apply rules analogous to those governing legal marriages:

  • Duties of cohabitation, cooperation, and support.
  • Duty of fidelity.
  • Sharing of "marital" expenses.
  • Joint liability for debts incurred for daily household matters.
  • Separate property principles for assets acquired individually.

The right to cancel inter-spousal contracts (Article 754) is debated but would likely be applied by analogy if the naien relationship is functioning.

III. The Complexities of Bigamous Naien (重婚的内縁 - Jūkon-teki Naien)

A particularly complex situation arises when a naien relationship exists while one of the partners is still legally married to another person. This is known as a "bigamous naien."

A. Conditions for Protection

Historically, courts were reluctant to protect bigamous naien relationships, often viewing them as contrary to public policy due to the principle of monogamy. However, the modern judicial trend is to afford protection to the naien spouse if certain conditions are met:

  1. The legal marriage must be irretrievably broken down: The existing legal marriage must be a mere "empty shell," with no prospect of reconciliation and often characterized by long-term separation (a state of jijitsujō no rikon - 事実上の離婚, or de facto divorce).
  2. The naien relationship must have the substance of a true marriage: The couple must be cohabiting with the intention of being spouses and sharing a life accordingly.

If these conditions are met, the law tends to prioritize the protection of the de facto family unit over the hollow legal marriage. For instance, the Hiroshima High Court (Matsue Branch) decision of November 15, 1965 (広島高裁松江支決昭和40年11月15日高民集18巻7号527頁) affirmed a claim for property division by a naien wife even though her partner was still legally married, finding the legal marriage had broken down.

When a bigamous naien exists, conflicts can arise concerning marital effects that might typically accrue to a spouse:

  • Property Division: As noted, naien spouses in a substantially broken legal marriage context can often claim property division from their naien partner.
  • Marital Expenses/Support: This is more complex. If the legal marriage is not entirely defunct and the legal spouse is still dependent, their claim for support from the shared partner is generally prioritized over that of the naien spouse. The Tokyo Family Court decision of August 20, 1969 (東京家審昭和44年8月20日家月22巻5号65頁) illustrated a situation where the court prioritized the needs of the legal wife and children of the legal marriage in allocating marital expenses.
  • Social Security Benefits: As mentioned, if the legal marriage is clearly broken down, the naien spouse who was the de facto dependent is often given preference for survivor benefits (e.g., Supreme Court decision of April 14, 1983 (最判昭和58年4月14日民集37巻3号270頁)).
  • Wrongful Death Claims: Claims for loss of support by a naien spouse may be recognized if dependency existed and the legal marriage was defunct. However, the legal spouse retains inheritance-based claims. Solatium claims would likely depend on which relationship was the substantive spousal one at the time of death.

IV. Effects Not Afforded to Naien Relationships

Despite the significant protections developed, naien relationships remain distinct from legal marriage in several crucial aspects:

  • No Automatic Change of Surname: Partners in a naien relationship retain their respective surnames.
  • No Attainment of Majority: Forming a naien relationship does not confer legal majority on a minor.
  • Children's Status: Children born to a naien couple are considered non-marital children (hi-chakushutsushi - 非嫡出子). The naien itself does not legitimate them; paternity must be established through acknowledgment (ninchi - 認知) by the father for him to have legal parental rights and obligations.
  • Parental Authority: For non-marital children, parental authority typically rests solely with the mother by default, unless the father acknowledges paternity and an agreement or court order establishes his parental authority.
  • No Creation of Legal Affinity: A naien relationship does not create legal ties of affinity with the partner's blood relatives.
  • No Statutory Inheritance Rights: This remains the most significant difference. A surviving naien spouse cannot inherit from their deceased partner's estate under intestacy laws.

V. Naien by Choice: "Jijitsukon" (事実婚)

In contemporary Japan, there is an increasing number of couples who consciously choose to cohabit and build a life together without formally registering their marriage. This is often specifically referred to as jijitsukon (factual marriage) when the decision not to register is deliberate. Reasons for this choice vary, including:

  • A desire for both partners to retain their respective surnames (as legal marriage requires adopting a single family surname).
  • A philosophical objection to state involvement in personal relationships.
  • A desire for a more flexible or less legally binding union.

The legal treatment of these "intentional" naien relationships generally follows the protective analogies developed for more traditional naien cases. However, if the explicit and clear intention of the parties was to avoid specific legal obligations that normally attach to marriage-like relationships, courts might be more circumspect in applying certain protections by analogy, especially if doing so would contradict the couple's expressed wishes.

VI. Conclusion

While Japanese law formally distinguishes between registered marriage and naien (de facto marriage), a substantial body of case law and specific legislative provisions have extended significant—though not identical—legal protections to naien relationships. This recognition is rooted in principles of equity and the need to address the social realities of stable, marriage-like cohabitation. Protections are most robust concerning the internal relationship (support, fidelity, property division upon breakdown) and certain third-party claims or benefits related to dependency.

However, crucial distinctions persist, most notably the absence of inheritance rights and the different legal status of children born to such unions. In cases of bigamous naien, the law navigates a complex balancing act, generally prioritizing the substantive family unit if the legal marriage is no longer viable. The ongoing evolution of societal norms regarding relationships continues to shape the legal discourse surrounding naien in Japan.