Criminal procedure holds the discovery of substantive truth as its highest value. However, this fact-finding function cannot be the sole aim of criminal procedure to be attained at all costs. Rather, it is constrained by the principles of due process and a timely trial. Therefore, although the Criminal Code provides for the crime of perjury which deters witnesses from hindering a fair trial with false testimony, the Criminal Procedure Code partially concedes the fact-finding function of criminal procedure by providing for the witness’s right to refuse testimony as a function of due process.This right to refuse testimony is enforced by the obligation to inform the witness on the existence of this right. The question is, if the judge questions a witness in violation of the right to refuse testimony and the obligation to inform, and the witness makes a false statement under oath, can the witness be punished for perjury? This is a question of weighing the values of substantive truth and the principle of due process when they are in contradiction, as due process is the source of the right to refuse testimony and, in certain circumstances, constitutes a limit on the ideal of substantive fact-finding.The decisions in this study take the position that the standard for finding a witness guilty on perjury should be whether there has been an actual hindrance to exercise the right to refuse testimony due to the failure to inform. In this sense, the cases give more weight to the due process considerations of witness examination than previous Korean Supreme Court cases. However, the studied cases are incorrect in limiting the affirmative defense to perjury to those cases where the failure to inform resulted in an actual hindrance to exercise the right. The witness is an individual who bears the obligation to appear at court even though it is not the witness’s own trial, swear a legally binding oath, and testify, all in the interest of substantive fact-finding. The Criminal Procedure Code obligates these witnesses to give testimony, but also gives them the right to refuse testimony where the witness may incriminate himself or herself or close family members. The obligation to inform the witness of this right forms a procedural safeguard to enforce the right. Therefore, the presiding judge’s failure to inform the witness of the existence of the right is a violation of due process and the testimony is given illegally, meaning the witness should not be found guilty of perjury even if the testimony was false.The obligation to inform the witness of the right to refuse testimony exists to guarantee the right to refuse testimony by reminding the witness of the right, thereby giving the witness the ample opportunity to reached an informed decision on whether to stay silent or testify. Therefore, the obligation to inform (Article 160 of the Criminal Procedure Code) guarantees the exercise of this right even when the witness does not know he or she has this right, or is ignorant of how to exercise the right. If the presiding judge violates this regulation and compels the witness to testify without informing the witness that the witness has the right not to testify, such an act on the judge’s part is far more than a minor infraction that has no effect on the legal existence of the crime of perjury. In the cases where the court has failed to inform the holder of the right to refuse testimony, there is no expectation that the witness in question will not commit perjury. It is worth noting that the newly amended Criminal Procedure Code now provides for the exclusion of illegal evidence, and due process is increasingly important at trial as well as during the investigative phase. Therefore, a failure to adhere to laws protecting the witness, especially the failure to inform the witness of a right to refuse testimony should be a full defense to the crime of perjury.