The Criminal Procedure Act of Korea prescribes that a written statement prepared by the investigative authority is admissible as evidence, only if it was prepared in compliance with the due process and proper method. The requirement of ‘due process and proper method’ means the legality of the process and method of preparing a written statement by the investigative authority. On the other hand, the Article 7 of the Korean act on protection of specific crime informants(hereinafter referred to as “the informants protection act”) provides that when any retaliation is likely to be taken against an informant of a crime or his/her relatives, prosecutors or police officers are not required to note all or part of information which verifies the identity of the informant of the crime, such as a name, age, address or occupation. In regard to the requirement of ‘due process and proper method’, it is questionable whether a written statement of witness under a pseudonym is admissible as evidence where it is prepared on the crimes other than the specific crime prescribed in the informants protection act. In this case, the lower court and the appellate court decided that a written statement of witness under a pseudonym was not admissible because the questioned crime of blackmail was not the specific crime and the statement under a pseudonym did not comply with the due process and proper method. On the contrary, the Supreme Court of Korea decided that the investigative authority could prepare the protocol of a written statement under a pseudonym, if there had been a probable cause considering a combination of circumstances such as the relation between witness and the defendant, the type of crime, the necessity of protecting the witness because the Criminal Procedure Act did not require a real name in the written statement to verify the identity of witness. And the Court ruled that the meaning of ‘due process and proper method’ prescribed in the Article 312 of the Criminal Procedure Act was that the investigative authority must comply with all process and methods provided in the Criminal Procedure Act, like informing of the right to remain silent where a written statement was prepared. However, the balancing test of a probable cause in the ruling has a problem with not presenting a bright-line rule in the investigative procedure and arousing new controversies.