For a long time, a majority of elected candidates in the national district for the upper house elections have not only been backed by interest groups but have been representatives of nationally-organized interest groups. Interestingly, in Japan, there are no laws requiring that upper house members be vocational representatives. How has the pattern “interest groups endorse and parties nominate” almost become a norm? Some analyses already point out that this is because interest groups with sound, vertically-integrated organizations have advantages in mobilizing votes and can redress parties’ mobilization weaknesses. While it is true that parties without sound local chapters have difficulties campaigning effectively, this paper argues that the pattern “interest groups endorse and parties nominate” should be understood in a broader context of clientelism where the LDP dominated Japan over a long period and, as a patron, used public policy to promise and deliver benefits in exchange for the electoral support of interest groups, namely, the client. However, a clientelistic exchange is contingency-based and is not simultaneous, and a monitoringmechanism is therefore needed to know whether or not the client voted for the patron as promised. Although lower house elections are critical, due to electoral rules, votes by different groups for a particular candidate cannot be easily investigated in lower house elections. By contrast, a nationwide district in relation to the upper house elections (1947-1980) has llowed nationally organized interest groups to show their ability to mobilize votes. As a result, the national district in regard to the upper house elections has inadvertently served as a monitoring mechanism for parties to investigate votes by different groups. This paper also investigates how both parties and interest groups have adapted themselves to the subsequent electoral rule changes of 1983 and 2001, and have continued to use a nationwide district in the upper house elections to serve as a monitoring mechanism. In conclusion, this paper discusses possible changes in the relationship between parties and interest groups in Japan that resulted from the change in government in 2009.