The Code of Criminal Procedure states that "when a judicial police official deems an offense has been committed, he shall investigate the offender and evidence thereof." Accordingly, the police are empowered to investigate not only penal code offenses but also all illegal acts punishable under Japan's judicial system. Most cases are investigated by the police and referred to the public prosecutor’s office for prosecution. While public prosecutors are also empowered to conduct investigations, their investigations are generally supplementary. The primary duty of the public prosecutor is to determine case dispositions and prosecute suspects.

Several other authorities such as the Japan Coast Guard and the Narcotics Control Department possess investigative powers, which are authorized by law. Their investigations are generally limited in scope and number

In addition to criminal investigations, the police perform a wide range of administrative activities to maintain public safety and order applying various acts such as the Road Traffic Act and the Anti-Boryokudan Act.

The police also maintain close contact with local communities to:
・Prevent crimes;
・Handle lost and found articles;
・Give guidance to juveniles;
・Help people in times of disaster;
・Provide care for lost children and runaways; and,
・Offer counseling services to help citizens solve their problems.