The purpose of this study is to examine the current state of the researches on the private investigation and to suggest the developments of related researches in response to legalization of the private investigation system. For such purposes, this study analyzed the preceding researches on the private investigation. The private investigation means to provide the security service in the private level and large numbers of people hoped for the legalization of the private investigation system because it would allow more high-quality security service. To meet such expectations, there have been numerous researches on claiming for introducing the private investigation system, examining the legislation for introducing the private investigation system, and introducing the overseas cases starting from the mid-2000s. However, the previous researches on the private investigation focused on the police studies, public administration studies, and legal studies. The previous researches also lack quantitative researches and bill-related researches. Therefore, this study identified such issues and proposed the improvement strategies.