Born in Kurume City in Fukuoka Prefecture, Koga left junior high school to be a painter, and he moved to Tokyo in 1912. He attended the institute of the Taiheiyoga-kai, and also the institute of the Japan Watercolor Painting Society the following year. In 1915, Koga entered the priesthood and studied Buddhism at Taisho University, but at the same time he continued painting and became a member of the Japan Watercolor Painting Society in1916. In 1917, he won the prize at the fourth exhibition of Nika-kai, and the next year he decided to concentrate on painting, and withdrew from the university. Koga was also interested in oil painting, and exhibited his oil work at the ninth exhibition of Nika-kai in 1922. In the same year, he created “Action”, the avant-garde group, with Kigen Nakagawa and other young contemporaries. In 1930, Koga was talked into joining the 1930-kyokai and entered it, but withdrew at once. He died in Tokyo.