A native of Nagano Prefecture, Koyama studied painting with Fujishima Takeji (1867-1943) and Yasui Sotaro (1888-1955), two of the most acclaimed Japanese painters working in the Western style. He spent most of the 1920s in Europe, adapting for his own paintings the representational style of Paris prevalent before the war. His work was shown in the Salon d'Automne in Paris in 1926 and, upon his return to Japan in 1928, his work was included in the exhibitions of the Nikakai, Shun'yokai, Issuikai (which he co-founded with Yasui Sotaro), Nitten, Nihon Geijutsuin, and Japan Art Academy. In 1970 he received the Cultural Merit Award. In the 1970s he established the Koyama Keizo Art Foundation to promote the study of art and oil painting restoration and, in 1975, the Koyama Keizo Museum opened in his birthplace, Komoro.