We documented the fluctuation of daily time budgets in Japanese black bears (Ursus thibetanus japonicus) throughout the year using continuous day-to-day data from activity sensors integrated into global positioning system collars, during 2003–2009. We evaluated the influence of availability of food resources on daily active time of bears in conjunction with differences in sex and reproductive status of females (with or without offspring). The daily active time of bears fluctuated nonlinearly throughout the year. There were 3 turning points at which the activity level clearly changed from increasing to decreasing, or vice versa. Level of activity gradually increased in spring and reached a peak in July, then decreased and reached a trough in late August, and increased and reached a peak again in October. Males and females exhibited similar patterns of seasonal fluctuation in daily active time, although there were differences of activity levels between sexes during some periods. Dates of the 3 turning points did not differ between sexes, or among years. Seasonal variation in food availability may explain the timings of the turning points, at least in part. In addition, yearly variation in food resources, especially hard mast, may have affected the increasing and decreasing pattern of daily active time during autumn. Our results suggest that evaluating activity level based on pooled data without examining differences within a season (or month), and differences in year, sex, and reproductive status may result in misinterpretation.