日本新党
Yamamoto was elected to the House of Representatives for the first time in 1993 as a member of Morihiro Hosokawa's Japan New Party but lost the seat in 2000. In 2001, he was elected to the House of Councillors for the first time. He was the expert of the social welfare, and wrestled with many problems over the social security system such as the reform of a pension policy and the medical system, the foundation of "the nursing care insurance". The question number of times in his Diet was the top-class. There is little legislation introduced by a Diet member in the Japan ("Act of Parliament").　However, he performed much an "Act of Parliament".