The multimember constituency system of the Japanese House of Representatives adopted in 1925 has lasted up to the present time, with the exception of a general election in 1946. This system is unique in the world and has influenced the electoral behavior of candidates as well as voters in Japan. It is inevitable that candidates from the same party are in competition.
One remarkable characteristic of the election results at the district level is that votes obtained by an individual candidate show a tendency to concentrate into the particular area. To examine the regional distribution of votes, we use the RS index and the DS index which was devised by Mizusaki, and trace the scores in the general elections from 1958 to 1993. The computed values have been decreasing year by year, but in some rural districts both indices show still very high scores.
The phenomenon of the concentration of votes in a particular area in a district has been considered as a reflection of premodern electoral behavior. Although this view cannot be denied in this paper, we try to explain the phenomenon in a different way. We would like to show that the spatial competitions aomng the plural candidates from the same party can be interpreted as a rational behavior with the strategy to win elections.

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