White Paper on Crime 2021 Part2/Chapter2
Judicial police officials are to refer every criminal case to public prosecutors, except  cases subject to disposition as trivial offenses (certain minor offenses committed by adults with particularly strong mitigating factors that satisfy public prosecutors’ predesignated criteria, are not required to be referred to public prosecutors pursuant to the proviso stipulated by Article 246 of the Code of Criminal Procedure) and  those of certain violations of the Road Traffic Act that are subject to Hansokukin (administrative fine).
Public prosecutors investigate cases referred by the police or other special judicial police officials. In addition, public prosecutors may institute an investigation, on their own initiative if necessary, or in response to a complaint or accusation directed to them. In concluding the investigation, they decide whether or not to prosecute a suspect, based on factors such as whether or not his/her act constitutes an offense and punishment is required on his/her case.