The natural history of an animal provides crucial information for more focused studies in areas such as ecology, ethology, and evolution. Dinodon orientale is a small, nocturnal, and rather rare endemic species in Japan. Here we provide information on the natural history of D. orientale including information on seasonal activity pattern, body size, body mass, and body condition index associated with sex, based on 110 individuals captured in Yamanashi Prefecture, central Japan. Both sexes demonstrated a similar trend in the seasonal activity pattern and body condition. The seasonal activity peak of both sexes was in September, probably because hatchlings appeared in September. Body condition was high at the beginning and end of the active season and low in summer. We did not observe sexual dimorphism in snout-vent length (SVL) or body mass. The seasonal trend in the body size distribution of captured snakes suggests that hatchlings of D. orientale (of around 180 mm SVL) appear in September, grow to an SVL of about 250 mm by late October, and begin hibernation in November. Juveniles emerge in April or May and grow to an SVL of around 350 mm by late September.