The purpose of this study is to investigate the socialist movements in Japan in the 1930s on the basis of the antiwar movements by the Japanese Communist Party and to make positive exploration into the space of peace in Japan in their militaristic past characterized by wars. The Japanese Communist Party began to expand its organization in the year of 1931 to acquire about 40,000 allies and create the largest organization of the prewar Communist Party. On the basis of the largest organization and mobility, the Japanese Communist Party launched antiwar movements against the army, along with antiwar activities and antiwar propagandas for the public. The National Council of Trade Unions in Japan, which was the strong extra-department body of the Communist Party, adopted the ‘abolition of the Japanese monarchism’ as its platform and associated labor disputes with the opposition to imperialist wars to fight. The Japanese Communist Party tried to raise a civil war through diverse antiwar struggles to start a socialist revolution between 1932 and 1933. It was a bold trial with the abolition of the Japanese monarchism as the platform. The party played a central role in spreading the anti-Fascist Popular Front for about a year between 1935 and 1936 and actually developed Popular Front activities in association with the Social Public Party, consequently enabling the Japanese society, whose resistance almost died out at that time, to make the final movements against wars. The antiwar movements by the Japanese Communist Party assisted the Japanese society to secure the entity of peace that denied wars in the 1930s when the wars and the support for the Japanese monarchism were spreading. The antiwar activities by the Japanese Communist Party can be meaningful, despite many limitations, in that they created the space of peace against the overwhelming Japanese nationalism and wars.