The breeding activity of the Japanese common toad, Bufo japonicus formosus, was monitored in Yamakita-machi, Kanagawa Prefecture and Minami-osawa, Hachioji-shi, Tokyo, during 1992–1994 and 2011–2013. The breeding activity began in late February-mid March, and lasted for one to three weeks. The number of breeding toads captured per day varied, and the effects of the lunar phase on the breeding activity were analyzed using circular statistics. Rao's spacing test indicated that large reproductive events were not uniformly distributed along the lunar cycle (U=166.1, n=46, P<0.01); more events occurred around the new moon and very few events occurred around the full moon. In addition, analysis of a generalized linear model incorporating both lunar phase and weather conditions, such as temperature and rainfall as predictors, indicated that the lunar phase significantly affected the breeding activity, even after taking into account the effects of weather conditions. The adaptive significance of this lunar-mediated breeding activity of the toads is discussed. Reproductive synchronization in this species may be related to the behavioral response to the lunar cycle.