This study aims to assess the effectiveness of the prosecution system granted only to public prosecutors (PPs), in regard to prosecution proceedings in accordance with the crime investigation policy of police in Japan. Using data generated by the Ministry of Justice (MoJ) and the National Police Agency (NPA) in Japan during the last half century, the trends of numbers of suspects referred and the rate of disposition, prosecution, summary trial procedure, suspended prosecution (details of which are described in the text), were calculated by type of offence including non-traffic penal code offences (NTPC), negligence in the pursuit of social activities on the occasion of traffic accidents (NPSATA), special law offences (SLO) excluding violations of road traffic related laws (VRTRL) and VRTRL. The results of the study indicated that there are three major issues about prosecution policies and practices by the PPs, namely: crime investigation activities; monopolising discretionary decisions with prosecution and suspended prosecution; and disposition of traffic related crimes. Practical implications of the present results are discussed in the context of crime investigation policy by police. (Published Abstract)