According to Criminal Procedure Act amended in 2020, Criminal Investigation Rules have been pre-announced. Recently, vigorous discussions on the rules are pending, but the discussions are mainly connected with ‘Readjusting Investigation Rights’. However, there are clauses concerning ‘Investigation Rules’ in the Criminal Investigation Rules in 2020, and discussions about this subject is needed. As amended Criminal Procedure Act gets into enforcement, control to interrogation phases will be much easier. However, considering the range of application of hearsay rule and ‘Evidence for Challenging Admissibility of Evidence’ clauses, precontrol to interrogation phases is necessary and important as ever. So this study explored Criminal Investigation Rules in connection with concerning clauses, and contemplated the investigation practices and previous researches about the subjects, and suggested the improvement methods. In chapter 2, the contents and circumstances of legislation were explored in connection with investigation, interrogation and interview clauses of Criminal Investigation Rules in 2020, and the meaning of Criminal Investigation Rules such as integration fo concerning rules and extension of the range of application of the rules were identified, meanwhile the problems such that clauses concerning recording of interrogation are poor by comparison with clauses concerning interrogation were also identified. In chapter 3, investigation practices and previous researches about formation of protocol concerning interrogation of criminal suspect according to formation routine of the protocol, and existence of various errors in almost all sub-phases of formaton of the protocol and necessity of improvement of Criminal Investigation Rules were identified. In chapter 4, improvement methods which consist of 18 improvement methods of concerning subjects were suggested along with provisions.