He studied law at Tokyo Imperial University and became a lecturer at a high school after graduation.

In 1922 he was sent to Europe to study law; there he familiarized himself with Marxism and joined the Communist Party of Germany.

Fukumoto wrote numerous articles and was well known in left-wing circles for his interpretations of Marxism-Leninism and his criticism of other Japanese Marxist scholars, especially Yamakawa Hitoshi and Kawakami Hajime.

His writing style was considered complex and he was more interested in the theoretical than the practical aspects of Socialism.

Eventually the Comintern took the view that this was happening and in 1927 it issued a thesis attacking both Yamakawa and Fukumoto and demanding that the party strive for an immediate two-stage revolution to overthrow the Japanese government, and especially the Emperor system and Diet of Japan, redistribution of wealth, and pursue a favourable policy with the USSR.