Koyama attended Takasago High School, and entered NPB at age 18, drafted by the Osaka Tigers.

[citation needed] Koyama won 20 or more games three straight years from 1958 to 1960; in 1959 he led the Central League in innings pitched, with 344.

In 1961, Koyama suffered his first losing season, going 11–22, leading the Central League in losses despite a stellar 2.41 earned run average.

Unfortunately, he lost two games in that year's Japan Series, as the Tigers fell to the champion Toei Flyers.

[2] On May 2, 1992, Tsutomu Seki discovered the minor planet 1992 JE, naming it "13553 Masaakikoyama" in honor of Koyama.