Preliminary investigation is not any more a professional jargon among Korean citizens. Both investigation entity and ordinary citizens differentiate this word from formal investigation and understand it as “investigatory activities to verify the crime information prior to initiating formal investigation made by enforcement entity. However, the word “preliminary investigation” has still been strange to academic circle, and does not find its place even in the Criminal Procedure textbook. In addition, its nature and concept have not been fully established, either. It makes a great difference in its application whether to regard the preliminary investigation as part of investigation or as having different concept or nature. Consideration of it as part of formal investigation would be subject to the strict application of the investigation procedures in Criminal Procedure Law. Currently, preliminary investigation appears to deviate completely from the scope of Criminal Procedure Law. The current practice appears that the investigation entity does not recognize the investigatee`s fundamental defense rights on the grounds that he/she is not booked despite stringent questioning toward him/her. Another problem arises out of conflicts involving directional hierarchy between prosecutors and policemen. While the Criminal Procedure Law requires the policemen to respect investigatory direction of prosecutors, the policemen vigorously refuse to follow the investigatory direction of prosecutors and forward a case to the prosecutors for the reason that it is not an investigation step preceded by booking procedure. While the prosecutors have disrespected the various miranda rights of an investigatee by employing the different concept and nature from formal investigation, the policemen also have imitated the prosecutor`s practices and made every effort to escape from the control and direction of prosecutors. This Paper is intended to analyse the concept and nature of the preliminary investigation and suggest the solutions to this issue. All the disputes involving preliminary and formal investigation find its roots in differentiating the legal nature of them. The simple solutions to this dispute can be easily identified. The author`s solution is the application of common jurisprudence to both investigation by adopting the common legal nature in them. In this way, the investigatee also can entertain the same procedural defense rights which a suspect can be entitled to. Furthermore, if an accusor is qualified to apply for ``petition for adjudication`` or judicial review after internal closing decision, both an accusor and an investigatee can be better protected under Criminal Procedure Law than in different legal standards. For the sake of preventing the disadvantages resulting from the application of same status, the current preliminary investigation practices should be maintained, such as prohibition of investigatory questioning toward an investigatee prior to booking, restriction on fingerprinting and formulation of investigation records, etc.