This was the first Japanese attempt at a semi-automatic pistol and was designed by a retired army officer, Colonel Kirijo Nambu.
The Model 04 (also known as the Nambu Type A) was initially intended for sale to civilian customers and to army officers, who at that time had to buy their own weapons. It is thought that the pistol's '04' designation probably refers to the fourth year of the Taisho reign - 1915. Yoshihito Taisho was the 123rd imperial ruler of Japan, from 1912 to 1926.
It was a locked breech weapon, with the breech being locked by a floating locking block attached to the barrel extension. As the barrel moved forward the block was lifted as it rode across the frame, forcing a lug upwards to engage and lock into a recess in the bolt.
The weapon had a grip safety but no holding-open lock, so that when the last round in a magazine had been fired the bolt was held open by the magazine lips, but when the magazine was removed the bolt then closed, needing to be recocked after the new magazine had been inserted. Sights were: fixed post frontsight; ramped notch backsight. Early models had a wooden holster stock which attached to the butt, but this was abandoned in 1912.
The Model 04 was adopted by the Imperial Japanese Navy and by the Thai Army, but was never formally adopted by the Japanese Army, although many officers bought one privately.