Saotomo was born in Harbin, Manchuria in Northeast China, and thus was raised in Manchukuo; however, following the defeat of Japan in World War II, he was evacuated to Kyushu in 1946.
He moved to Tokyo in 1948, and attended Keio University's Literature Department, but left school before graduating.
In 1956, together with other like-minded authors, he formed a literary criticism group called Shosetsu Kaigi ("Fiction Conference"), with members supporting each other by reviewing each other's work.
One of the products of this collaboration was his novel Kyojin no Ori, about the María Luz Incident, which was awarded the 60th Naoki Prize (1968下).
After contracting stomach cancer, Saotome died at a hospital in Kamakura, Kanagawa, Japan.