Katayama was a Christian and a member of the Japan Socialist Party (JSP) and leading person within the Japanese Christian socialism. In 1930 , he was for the JSP in the Japanese diet (Parliament) chosen. In 1940 , he resigned in protest. After the Second World War was Katayama in 1946 President of the Japanese Socialist Party. At the first democratic elections in Japan after the war he became Prime Minister of Japan. He joined back In 1948 .
Katayama belonged to right wing of the JSP. Later he stimulated the creation of the Democratic Socialist Party (DSP).