Unjust Enrichment in Japanese Law

Introduction

The law of unjust enrichment (futo ritoku) in Japan is governed by Articles 703–708 of the Civil Code. The general provision, Article 703, states: “A person who has benefited from the property or labour of another without legal cause and has thereby caused loss to another shall be obliged to return such benefit to the extent of the benefit existing.”

The unjust enrichment claim occupies a distinctive position within the Japanese law of obligations. It is subsidiary to contractual and tort claims — a plaintiff who has a claim in contract or tort cannot circumvent the rules governing those claims by bringing an action in unjust enrichment. However, where no contractual or tort claim is available, unjust enrichment provides a residual remedy that prevents one party from being unjustly enriched at the expense of another.

The Requirements of Article 703

Japanese doctrine identifies four requirements for a claim under Article 703.

Enrichment (Ritoku)

The defendant must have been enriched, meaning that the defendant’s economic position has improved, or that the defendant has avoided an economic loss that would otherwise have been incurred. Enrichment can take the form of an increase in assets (receipt of money, property, or services) or a decrease in liabilities (discharge of a debt, release from an obligation).

Impoverishment (Sonshitsu)

The plaintiff must have suffered an impoverishment, meaning that the plaintiff’s economic position has worsened. The impoverishment must correspond to the defendant’s enrichment — it is not necessary that the impoverishment and enrichment be identical in form, but there must be a correlation between them.

Causal Relationship

There must be a causal relationship between the enrichment and the impoverishment. The enrichment must have been obtained “at the expense of” the plaintiff. This requirement is satisfied where the plaintiff’s property or labour was the direct source of the defendant’s enrichment, or where the enrichment was obtained through an infringement of the plaintiff’s rights.

Absence of Legal Cause

The enrichment must have occurred without legal cause (hōteki genin). This requirement excludes cases where the enrichment is justified by a contract, a statute, a court order, or some other legally recognised basis. Where a valid contract exists between the parties, the enrichment is legally caused and Article 703 does not apply — the parties’ rights are governed by the contract.

The Subsidiary Nature of Unjust Enrichment

Japanese law treats the unjust enrichment claim as subsidiary (hojūteki) to other claims. Where the plaintiff has an available claim in contract (Article 415) or tort (Article 709), the plaintiff must pursue that claim and cannot alternatively bring an action in unjust enrichment to avoid the limitations applicable to the primary claim.

However, the subsidiary nature of unjust enrichment has been qualified in several respects by the courts. Where the contractual claim has been extinguished by the running of a limitation period, the plaintiff may still bring an unjust enrichment claim if the underlying obligation has not been performed (the theory of the “survival of the claim after prescription” ). Similarly, where a tort claim is unavailable because the defendant’s conduct was not negligent, the plaintiff may still recover in unjust enrichment if the defendant has been enriched at the plaintiff’s expense.

Types of Unjust Enrichment

Japanese legal scholarship, drawing on German doctrine, distinguishes three principal types of unjust enrichment.

Performance-Based Enrichment (Condictio Indebiti)

The condictio indebiti — the claim for the return of a performance made in error — is the paradigmatic case of unjust enrichment. Where the plaintiff transfers property or makes a payment to the defendant in the mistaken belief that the plaintiff is obligated to do so, and the obligation does not in fact exist, the plaintiff may recover the property or payment under Article 703. This claim is available where:

• The plaintiff performed under a mistake of law or fact
• The performance was not due (the obligation did not exist)
• The mistake was not caused by the plaintiff’s gross negligence (although mere negligence does not bar the claim)

Enrichment by Infringement (Eingriffskondiktion)

The Eingriffskondiktion — enrichment by interference with another’s rights — arises where the defendant acquires a benefit by infringing the plaintiff’s rights. The paradigmatic case is the unauthorised use of another’s property: where the defendant uses the plaintiff’s land, intellectual property, or other asset without authorisation, the defendant is enriched by the value of the use, and the plaintiff is impoverished by the loss of the opportunity to exploit the asset.

The Japanese Supreme Court has recognised the Eingriffskondiktion in several contexts, including the unauthorised use of a patent (the defendant must account for profits), the unauthorised occupation of land (the defendant must pay mesne profits), and the infringement of personality rights (the defendant must disgorge profits obtained from the unauthorised commercial use of the plaintiff’s name or image).

Enrichment from a Situation (Lage- und Zustandskondiktion)

The Lage- und Zustandskondiktion — enrichment from a situation — covers cases where the defendant is enriched by circumstances for which the plaintiff is responsible but which do not involve a performance by the plaintiff or an infringement of the plaintiff’s rights. This category includes cases where the plaintiff incurs expenses that benefit the defendant (the defendant’s property is improved by the plaintiff’s expenditure) and cases where the plaintiff pays the defendant’s debt.

Article 704: Bad Faith Enrichment

Article 704 imposes a stricter standard on a defendant who received the enrichment in bad faith (aku’i). A bad faith recipient must return the benefit with interest from the date of receipt and is liable for damages if the benefit cannot be returned. By contrast, a good faith recipient (Article 703) is obliged only to return the benefit “to the extent of the benefit existing” at the time of the claim.

The distinction between good faith and bad faith is determined by the recipient’s knowledge of the absence of legal cause. A recipient who knew or ought to have known that there was no legal basis for the enrichment is treated as a bad faith recipient. The burden of proving good faith rests on the recipient.

Illegal Contracts: Article 708

Article 708 provides that a person who has performed under an illegal contract may not recover the benefit conferred. This rule — which excludes unjust enrichment claims where the underlying transaction is contrary to public order or good morals (Article 90) — reflects the principle that the courts will not assist a party who has participated in an illegal transaction.

However, the courts have recognised several exceptions to the Article 708 bar. Recovery is permitted where:

• The illegality is minimal and the plaintiff is not the primary wrongdoer
• The plaintiff was exploited by the defendant in the illegal transaction
• Denying recovery would produce a result that is contrary to the purpose of the prohibition (for example, where forfeiting the benefit would punish the innocent party more severely than the wrongdoer)
• The illegal transaction has been fully executed and the defendant is seeking to avoid performing a counter-obligation

The Equitable Nature of Unjust Enrichment

Japanese courts and scholars consistently emphasise the equitable character of the unjust enrichment claim. The remedy is not available as of right but is subject to the court’s assessment of the equities between the parties. The defendant may raise equitable defences, including:

• Change of position: the defendant who, in good faith, has changed position in reliance on the enrichment (for example, by spending the money received) may be relieved of the obligation to return the enrichment
• Estoppel: the plaintiff who has represented that the enrichment was intended may be barred from later claiming its return
• Unclean hands: the plaintiff who has acted wrongfully in connection with the enrichment may be denied recovery

Relationship with Restitution

The relationship between unjust enrichment and restitution in Japanese law is complex. While unjust enrichment provides a cause of action (the right to claim the return of a benefit), restitution describes the remedy (the return of the benefit or its value). Japanese law has expanded the availability of restitutionary remedies in the post-war period, particularly in the context of public law: where a government agency has collected a tax or fee without legal authority, the taxpayer may recover the payment under the unjust enrichment provisions, and some courts have awarded interest on the amount recovered as a form of restitution.

Conclusion

Unjust enrichment in Japanese law is a flexible and equitable doctrine that prevents one party from being enriched at the expense of another without legal justification. Its subsidiary nature, its classification into distinct types, and its responsiveness to equitable considerations give it a distinctive character within the Japanese law of obligations. The continuing development of the doctrine — particularly in the contexts of intellectual property infringement, public law restitution, and illegal contracts — demonstrates its vitality as a source of legal obligation.