Seasonal activity of the Japanese red-bellied newt, Cynops pyrrhogaster, was monitored in Minami-ohsawa, Hachioji-shi, Tokyo, from 2016 to 2017. The number of newts in the water of a pond was counted immediately after sunset, at least once a week, except in winter. The number of newts counted varied daily, and the time-series data on newt activity were analyzed using state-space modeling to estimate the impact of weather conditions and lunar phase on the activity due to the presence of plausible temporal autocorrelation. We found that some weather conditions, such as water temperature and atmospheric pressure, had significant positive effects on newt activity. Newts became more active during nights with higher temperatures and/or higher pressure. However, we did not detect any significant effect of the lunar phase on newt activity. The most important variable affecting newt activity was water temperature. The active season of newts was estimated to be nine months from March to November. Newt activity showed a sharp decline in the nightly activity of males and an extreme female-biased sex ratio was observed in early April suggesting that the degree of nocturnality differed across seasons and the two genders. This phenomenon may be related to the presence of mating activity during this particular season.