In September, police found one pistol and 11 rounds of ammunition in the locker rented by 43-year-old Masahide Sato.
Sato, who has been charged with violating the Swords and Firearms Control Law, admits to the allegations.
Police had previously arrested Sato for the alleged involvement in a bank-transfer fraud in Funabashi City, Chiba Prefecture. During the investigation, police found the key to the rental room.
Police suspect that Sato had intended to resell the weapon and ammunition.