This paper aims to examine how ideological conversion was launched and developed in association with Japanese monarchism through statements by judicial bureaucrats. As Japanese Communist Party came to be expanded, the governmental policy on communists of exclusive punishment changed. Judicial bureaucrats thoughts that they could theoretically criticize them by Monarchism, so they tried to convert their political creed through Monarchism. The first attempt was by Hirata Isao, a prosecutor, in 1929. He tried to convert Communist of Labour urging that Monarchism was the utmost social and political value to be protected by all means in Japan. As a result, the communists of Labour was persuaded to convert their politics. The consequence was that a new communist movement of Labour failed, while the bureaucrats were encouraged to launch a conversion system to communists as a whole making use of Monarchism established since Meiji Reform. A large scale of conversion was realized, for instance, mitigation of prosecution on communists in 1930 and suspension of prosecution in 1931, which turned out to be very effective means of conversion. However, shortly the early conversion policy changed to be a very oppressive one. The turning point was the conversion declaration by Sano Manabu and Nabeyama Sadachika in 1933. The direct reason of their conversion was the Manchurian Wan Owing to the Manchurian War, the Japanese people united ideologically worshiping their monarch. To their disappointment the Japanese people's loyalty was too strong, so they had no other choice to convert. Their conversion was just the start of the period of conversion in a large scale. Actually the conversion policy was experimented on Communists of Labour, and its success encouraged the bureaucrats and the policy was largely supported by the Machurian War. Moreover, the conversion of Sano and Nabeyama had much influenced social resisting groups and gave the bureaucrats a sort of confidence. Since the conversion of a large scale, judicial bureaucrats collected a lot of material on convertors, which enable them a psychological research on how and why they came to be communists. A very basic condition of conversion is, without doubt, imprisonment, and another reason of conversion is their family. But the burearcrats concluded that so called a national recognition is a true road to conversion. The burearcrats believed that a nation and state made of union of blood cannot be defeated and is a permanent life. Even a communist cannot deny that he or she is Japanese, thus they believe that communists would accept that they are subjects of Japanese monarch and identify with the monarch. On this ground the bureaucrats was confident that even non-converting communist shall turned out to be true Japanese in the long run. In this respect, the theory of ideological conversion was accepted as a truth which cannot be doubted or denied by the bureaucrats. Renunciation of practice of movements in the early period of conversion changed to be renunciation of revolutionary thoughts in the late 1933 which was stated in official documents.