Born in Hiroshima Prefecture, Nakai studied philosophy at Kyoto University, particularly aesthetics under Yasukazu Fukuda. He started the dōjinshiBi hihyō in 1930, which changed its name to Sekai bunka in 1935. He became a lecturer at Kyoto University while being active in left-wing social movements, protesting Japan's tilt towards fascism and promoting popular forms of culture through such concepts at the "logic of the committee." He founded the popular culture tabloid Doyōbi (Saturday) in 1936, but was arrested for his activities in 1937 and lost his university position. After World War II, he continued his political activism by teaching philosophy as part of the Hiroshima Culture Movement and by running for governor of Hiroshima Prefecture, only losing by a narrow margin. He was appointed the first Vice Librarian (fukukanchō) of the National Diet Library in 1948.