He was known for the development of naval gymnastics, which were meant to improve the alertness and flexibility of sailors in closed areas of warships and he was known for leading the paratroopers.

As a teacher of gymnastics at the Naval school, he had long felt a need that those in the navy should attain more alertness and flexibility of actions in the limited spaces of warships and he studied various methods; traditionally the Japanese Imperial Army had employed the Swedish gymnastics.

[2] Prior to the beginning of the Pacific War, he was appointed as a special commander, and on 11 January 1942, his troop successfully performed an airborne operation at Manado, North Sulawesi, Indonesia.

Earlier, he had been the leader in training paratroopers at Tateyama, Chiba, during which a number of trainees died due to various reasons.

[citation needed] According to Japanese reports, Horiuchi treated the inhabitants of the island well, and was appreciated them.

During his short tenure, he gave reportedly provided necessary goods such as salt, and reduced the tax to 1/4 of the previous amount.

Horiuchi reportedly used the local language and according to one Japanese source, treated the prisoners of war (POWs) of the Netherlands fairly.

[4] On 29 January 1948, a written indictment was read at the Manado Temporary Military Court which presented charges against Japanese Navy Colonel Toyoaki Horiuchi by the order of the Prosecutor General (dated 19 January 1948).

The charges were that Horiuchi ordered or allowed Dutch army officers to be subjected to systemic terrorism.

In January 1942, when he set up the parachute troop center, he strongly banned violence to those on the Dutch side.