In January, 1906, the government approved the 'Nihon Heiminto' (Japan Commoner Party) founded by Kojiro NISHIKAWA and his comrades, which was followed by the formation of the 'Nihon Shakaito' with Toshihiko SAKAI as its main founding member; the next month these two parties joined together to form a new party retaining the party name Nihon Shakaito in February, 1906.
同年1月、西川光二郎らによる「日本平民党」の結党が認められたのに続いて堺利彦らを中心に「日本社会党」が結成、翌2月には両党が合同して日本社会党が結成された。