  We study rank-size distribution of cities in Japan on the basis of data
analysis. From the census data after World War II, we find that the rank-size
distribution of cities is composed of two parts, each of which has independent
power exponent. In addition, the power exponent of the head part of the
distribution changes in time and Zipf's law holds only in a restricted period.
We show that Zipf's law broke down due to both of Showa and Heisei great
mergers and recovered due to population growth in middle-sized cities after the
great Showa merger.
