An Osaka police officer left his loaded pistol in the toilet of a convenience store in the city on Saturday morning but it was found shortly after by a cashier.
The officer, in his 20s, was on duty when he used the toilet at around 7:10 a.m. but left his gun behind when he left, the police said.
A store staffer found the gun about 20 minutes later and reported it to another officer who was on duty at the nearby U.S. Consulate General.
"We will try to prevent such incidents by handling our equipment more carefully," Tsuneo Fujii, an official in the Osaka Prefectural Police's security division, said in a statement.