Abstract
The purpose of this paper is to examine the institutional effect of parallel system on the party system and candidate system in Japan. It has passed almost 20 years since the new electoral system was introduced as the electoral system for the House of Representatives. From 1996 the election under the parallel system was carried out 6 times. In this paper, influence on the party system and the candidate system using the concept of effective number of party and candidate proposed by M. Laakso and R. Taagepera is examined. The fi ndings of this research show that the Duverger's law is functioning until the 45th general election in 2009.
