In a 1925 plan for a five-line subway system, the Marunouchi Line was planned to run from Shinjuku to Ōtsuka via Hibiya, Tsukiji and Okachimachi, as a underground route. A segment between Akasaka-mitsuke and Yotsuya began construction in 1942, but was abandoned in 1944 as a result of the continuing effects of World War II. On December 7, 1946, the Marunouchi Line was revised to begin from Nakano-fujimichō to the Mukōhara neighbourhood in Toshima Ward via Kanda and Ikebukuro, for a total length of . On March 30, 1951, a groundbreaking ceremony was held at Ikebukuro Station East Exit to begin construction of the initial segment of the Marunouchi Line.