What must be proved before a person is convicted as a co-perpetrator? Decision 2018Do20415 on joint criminal intent and functional control of the act

Question

Article 30 of the Criminal Act provides that where two or more persons jointly commit an offence, each is punished as a principal of that offence. Article 308 of the Criminal Procedure Act provides that the probative force of evidence is left to the judge's free judgment. The question is what must be established before a person is convicted as a co-perpetrator under Article 30: what mental element the person must have towards the offence others carried out, what part the person must have played in the carrying out, and how firmly the prosecution must prove both.

Decision 2018Do20415 raises that question in an appeal by the prosecutor. The first-instance court had convicted the defendants over trading and index manipulation on the KOSPI 200; the Seoul High Court reversed and acquitted, and the Supreme Court was asked whether that acquittal rested on a misunderstanding of the law on co-perpetition.

Material to check

The statutes are Article 30 of the Criminal Act, which punishes each of two or more persons who jointly commit an offence as a principal of that offence, and Article 308 of the Criminal Procedure Act, which leaves the probative force of evidence to the judge's free judgment. As published for this brief, Article 30 was last amended by Act No. 22007 of 29 September 2026 and Article 308 by Act No. 22009 of the same date; the decision applies Article 30 and Article 308 as the provisions governing co-perpetition and the evaluation of evidence.

The decisions are Decision 2018Do20415 (21 December 2023) and the two it cites. Decision 2002Do5112 (11 March 2005) is a case where a general hospital charged patients excessive medical fees and the finding of the court below, that the hospital director could not be seen as having conspired with staff to obtain the money by deception even though he had a rough awareness of the overcharging, was upheld. Decision 2017Do21033 (11 May 2018) is cited for the requirements for the establishment of co-perpetition and the degree of proof of whether it is established.

Possible readings

The decision reads Article 30 as having two elements. The subjective element is joint criminal intent; the objective element is the fact that the crime was carried out through functional control of the act, exercised on the basis of a joint decision. Both are needed, and the decision states them as the conditions for co-perpetition to arise.

On the content of joint criminal intent, the decision is explicit: it is not enough to recognise another person's wrongdoing and, without stopping it, tolerate it. The intent must consist in becoming one with others in order to commit a particular offence and, by using the acts of the different persons, carrying one's own will into effect. Drawing on Decision 2002Do5112 and Decision 2017Do21033, the decision requires the positions and roles of each person and the content of any encouragement of accomplices to be examined concretely across the whole course of the carrying out, and the resulting mutual-use relationship to be proved to the point where there is no room for reasonable doubt.

On the degree of proof, the decision states that where that proof is absent, the accused must be judged in the accused's favour even if a suspicion of guilt remains. Article 308 leaves the probative force of evidence to the judge, but the decision treats the outcome as reviewable within that framework: the reasons of the court below are checked against the relevant legal principles and the record for a failure to conduct the necessary deliberation, for departing from the limits of free evaluation by violating the rules of logic and experience, and for misunderstanding the law on conspiracy and functional control or omitting reasoning that affected the judgment.

What the decisions held

Decision 2018Do20415 (21 December 2023) dismissed the prosecutor's appeal in its entirety. The court below, the Seoul High Court in Decision 2016No603 (12 December 2018), had reversed the first-instance judgment, which had found the charges proved and convicted, and had acquitted, holding that the evidence submitted by the prosecutor did not prove beyond reasonable doubt the matters it listed. As the decision records, the conduct at issue was that of the first-instance co-defendants, who were said to have built speculative positions profiting from a fall in the KOSPI 200 index and to have manipulated that index; the question was whether that conduct could be attributed to Defendant 1. The panel was presided over by Justice Min Yu-suk, with Justice Chun Dae-hyup as reporting Justice.

The court below reasoned, as the decision records, that it could not be found proved beyond reasonable doubt that Defendant 1 recognised and tolerated the fact that the first-instance co-defendants were building those speculative positions and manipulating the index, or that Defendant 1 acted with joint decision-making with them, exercising functional control of the act through an essential contribution to the offence, such as building speculative positions or supplying the ideas needed for the manipulation.

Reviewing those reasons against the legal principles and the record, the Supreme Court found no such defect: no failure to conduct the deliberation needed for the decision below, no violation of the rules of logic and experience taking the matter outside the limits of free evaluation, and no misunderstanding of the law on conspiracy and functional control in co-perpetition, nor omission of reasoning, that affected the judgment. The appeal was therefore dismissed and the acquittal stood.

Points left open

In describing what the court below found unproved, the decision's reasons name the building of speculative positions and the supply of ideas needed for the manipulation as forms of essential contribution to the offence. The decision does not say how substantial a contribution must be before it counts as essential, and it does not draw the line in other fact patterns between a person who merely failed to stop an offence and a person who used another's act to carry out a joint plan. Those boundaries remain to be set case by case.

The decision also leaves open how the requirement of proof is applied to individual participants in a group offence. It directs that the positions and roles of each person and the content of any encouragement of accomplices be examined across the whole course of the carrying out, and that the mutual-use relationship be proved beyond reasonable doubt; it does not say what evidence of role or encouragement suffices for each person, and its review of the acquittal was confined to whether the court below's assessment crossed the legal limits it recites.

When to reconsider

We revisit this brief if Article 30 of the Criminal Act or Article 308 of the Criminal Procedure Act is amended in a way that changes the elements of co-perpetition or the evaluation of evidence, or if a later Supreme Court decision sets the threshold of contribution at which a person's part becomes functional control of the act, or distinguishes tolerance from joint criminal intent on particular facts.

Statutes referred to

• 형법 제30조
• 형사소송법 제308조

Decisions referred to

• 대법원 2023. 12. 21. 선고 2018도20415 판결 · Decision ledger
• 대법원 2005. 3. 11. 선고 2002도5112 판결
• 대법원 2018. 5. 11. 선고 2017도21033 판결

Verification status

The texts read for this brief were read at the National Law Information Center on 24 October 2026. They include the decision in its holdings, cited provisions and full reasons, and the two cited decisions in their published holdings and, for one of them, its summary.