The Peace Preservation Law (Japanese: 治安維持法; Chian-ijihô) was a Japanese law passed in 1925 as a mechanism for the royal family to entrench itself against a growing left wing. It forbade conspiracy or revolt against the kokutai ("national essence") of Japan, and effectively criminalized socialism, communism, and other ideologies that would threaten Japan's emperor-centered social order.
In 1920, a Tokyo Imperial University professor named Morito Tatsuo was prosecuted for publishing an article critical of Peter Kropotkin, and spent three months in jail on charges of treason. His case set a precedent in Japanese law that effectively criminalized the discussion of ideas, and the government's clampdown on dissent only intensified after the 1921 assassination of prime minister Hara Takashi. The Peace Preservation Law was therefore only a legislative embodiment of a legal superstructure that had already existed.
Japanese police that arrested more than 50,000 citizens over the next two decades, driving the Japan Communist Party and Korean Communist Party underground. The Special Higher Police were responsible for monitoring films and political campaigns, while military police watched affairs on the ground. This system remained in effect until the new Police Law was passed in 1947 under the eye of the American occupation of Japan. By then, the Peace Preservation Law had become unconstitutional under the new Japanese constitution.
