The penalties for a Japanese police officer losing a gun vary depending on the specifics of the situation. For example, if no serious consequences are caused and the gun is promptly recovered, the officer usually receives internal disciplinary action, such as suspension or a warning.
When Shirata Masahiro reported promptly, his superiors considered that no serious consequences had occurred, and he was demoted according to regulations.
He had worked hard to become a criminal police officer and achieved some accomplishments at the station, but it all turned to nothing.
To make matters worse, it was now discovered that his police gun was linked to a death. Even if it was a suicide by the person involved, the officer in question would bear criminal responsibility. Saying his life was ruined might be an exaggeration, but his career as a police officer was certainly over.
This shows how much courage Shirata Masahiro needed to confess the truth.
