Police read out the stalking victim's personal information when serving suspect Hideto Kozutsumi, 40, with an arrest warrant, The Mainichi Shimbun reported Monday.
Stalking victim Rie Miyoshi, 33, had asked police not to give her married surname and address to Kozutsumi when she reported receiving harassing emails from him.
After arresting Kozutsumi in June 2011 in Tokyo, they again gave out the woman's full name and address while questioning him, investigators found.
Kozutsumi later used that information to kill the woman at her residence, investigators said. Kozutsumi then killed himself.
While police officers followed procedures set out in Japan's Code of Criminal Procedure, "in light of the latest incident, we'd like to consider how we pass on and list up allegation details from the perspective of protecting the victim," said Masami Yamaguchi, deputy head of the police station where the suspect was questioned.