The Dropout(1982)

Detective Seiji Otaki is determined to find the psychopathic killer of a young woman who was ostensibly a student but in reality a high-priced prostitute. Even though he has been taken off the case for beating up a suspect, he refuses to let it go and recruits his mistress to act as a decoy for the killer. Her involvement turns out to be a fatal mistake, and when her husband gets out of prison, Detective Otaki is in worse trouble than ever.