The Japanese black bear (Ursus thibetanus japonicus) is a forest-dwelling species and can be very difficult to observe directly in most of its habitats. Therefore, as a research tool to evaluate activity patterns and time budgets, we fitted GPS radiocollars with activity sensors on 4 Japanese black bears, 2 males and 2 females, during 2003–06 in the Ashio Mountains, which have open habitats. Through comparison to observations from a video camera with activity sensor values, we categorized activities as either resting (inactive) or feeding with short distance movements (active). Bears were mostly diurnal, although they were increasingly active at dusk. Mean time inactive was 66%, including considerable inactive time during daylight. Time active ranged from 22–48%, depending on the bear. For one adult female, diel active time during a year she was solitary declined significantly from the previous year in which she had nursed 2 yearlings. For one sub-adult male, diel active time decreased significantly with increasing age. Although more research is needed to relate activity values to age and sex in a variety of habitats, we conclude that GPS collars with a built-in activity sensor can be an effective research tool for clarifying the living habits of Japanese black bears. If future studies can further refine relationships between sensor values and activity categories, diel energy costs for Japanese black bears may also be estimated.