The effects of rainfall events on soil CO2 fluxes were examined in a cool temperate Quercus/Betula forest in Japan. The soil CO2 fluxes were measured using an open-flow gas exchange system with an infrared gas analyzer in the snow-free season from August 1999 to November 2000. Soil CO2 flux showed no significant diurnal trend on days without rain. In contrast, rainfall events caused a significant increase in soil CO2 flux. To determine the effect of rainfall events and to evaluate more precisely the daily and annual soil carbon flux, we constructed a multiple polynomial regression model that included two variables, soil temperature and soil water content, using the soil CO2 flux data recorded on sunny days. Daily soil carbon fluxes on sunny days calculated by the model were almost the same as those determined by the field measurements. On the contrary, the fluxes measured on rainy days were significantly higher than those calculated daily from the soil carbon fluxes by the model. Annual soil carbon fluxes in 1999 and 2000 were estimated using models that both do and do not take rainfall effects into consideration. The result indicates that post-rainfall increases in soil CO2 flux represent approximately 16–21% of the annual soil carbon flux in this cool temperate deciduous forest.