In principle, an act of a third party violating or interfering with the maintenance of the marital life, which is the essence of marriage, by committing an unfaithful act with one of the spouses and inflicting mental suffering by violating the spouse's rights, constitutes an unlawful act, and the third party has a duty to compensate for the mental suffering suffered by the spouse as a result (Supreme Court, 2011M2997 en banc decision, decided on November 20, 2014).

Therefore, if a spouse has an affair with a third party during marriage, the other spouse can claim alimony against the third party, that is, the incestator.

The amount of alimony in commercial litigation is not determined uniformly, but is determined based on the specific case, taking into account various circumstances, such as the extent of the misconduct and the degree of damage.

Issues that the court considers when calculating the amount of alimony include:

• Marriage period and family relationship of the couple
• Duration and extent of misconduct
• The impact of cheating on a couple’s joint life
• Degree of mental suffering caused by cheating
• Circumstances after the misconduct was discovered (attitude shown by the adulterer)
• Whether the adulterer was aware of the other party's marriage, etc.

The following materials can be used as evidence to prove the misconduct of an adulterous spouse.

• Messages exchanged between spouse and adulterer (text, KakaoTalk, SNS, etc.)
• phone call recording
• CCTV footage of people entering and exiting accommodations such as hotels and motels or homes
• Photos of people together having skinship
• Card payment history
• Vehicle black box video
• navigation destination

Fraud is not necessarily proven with only one conclusive piece of evidence, and is often judged by combining multiple circumstantial evidence. Therefore, even if there is no direct evidence of sexual intercourse, misconduct can be recognized based on the relationship between the two people, the content of the contact, the type of meeting, etc., so it is important to systematically organize the available data.

Meanwhile, evidence collected through illegal methods such as illegal tailing, wiretapping, and home invasion may be legally problematic and may result in separate criminal liability. Therefore, it is important to collect evidence through legal procedures and methods rather than trying to secure evidence unreasonably.

Claims for alimony against incest must be filed within a certain period of time, and it may be difficult to file a claim after the statute of limitations expires, so pay attention to the period.

Statute of limitations for claims for damages arising from illegal acts (3 years from the date of knowledge, 10 years from the date of occurrence)

In principle, the misconduct of an adulterer constitutes a tort, so a lawsuit must be filed within 3 years from the date of knowledge of the misconduct and 10 years from the date of the misconduct (Article 766 of the Civil Act).

When filing an incest lawsuit after divorce (3 years from the date of divorce)

If a claim for alimony is filed due to divorce due to the breakdown of the marriage due to the infidelity of the adulterer, the lawsuit must be filed within 3 years from the date of divorce.

Regarding this, our court said, “The right to claim alimony on the grounds of divorce is a right to claim compensation for the mental pain suffered by the spouse when the marriage relationship breaks down and divorces due to the fault or illegal act of one of the spouses. Since it is reasonable to assume that the spouse was aware of the damage and the perpetrator at the time the marriage was dissolved, the short-term statute of limitations stipulated in Article 766, Paragraph 1 of the Civil Act is completed only after 3 years from then (Supreme Court, January 29, 2026, ruling 2025M10716).”