Many tree species have annual fluctuation of flowering and seed production. But there are few long-term and quantitative data of dioecious species. Cercidiphyllum japonicum Sieb.et Zucc. is deciduous riparian tree distributed in cool-temperate zone in Japan. C. japonicum is dioecious species. Seed of C. japonicum is very small and anemochory. The purpose of this study is to clarify the annual fluctuation of flowering and seed production, and the difference in flowering between female and male for long term. Research site is a riparian forest in Chichibu Mountains of central Japan. We set twenty seed traps in this plot, and measured the seed production from 1995 to 2006. And we checked the flowering of 60 individual from 2000 to 2007 by binocular. Tree sizes (DBH) were significantly different between reproductive and immature trees, but there was no difference in size between female and male trees. All reproductive trees were larger than 26 cm in DBH. Male trees showed the much flowering every year without clear annual fluctuation. On one hand, female trees also showed the flowering and seed production every year with annual fluctuation. As for C. japonicum, flowers have continued to bloom in large quantities every year without regard to sex. C. japonicum may select the mass dispersal of seeds every year through the miniaturization of seeds.