The diel activity pattern is a key factor in the coexistence mechanism of sympatric wild animals, enabling temporal niche partitioning. Although previous studies on sympatric ungulates (sika deer, Japanese serow, and wild boar) have reported dietary and spatial niche partitioning, temporal niche partitioning in these sympatric ungulates is not well understood. In this study, we aimed to elucidate the seasonal diel activity patterns of three sympatric ungulates in Gifu Prefecture, Japan, where they have been sympatrically distributed since 1978. We placed 21 camera traps from August 2017 to November 2019, and investigated diel activity patterns and photographic frequencies over three time periods (daytime, night-time, and twilight). Although we found no seasonal changes in diel activity patterns of three ungulates, but patterns showed differences among species. While sika deer showed crepuscular activity, wild boars and Japanese serows showed nocturnal activity. In addition, Japanese serows showed temporal niche partitioning against sika deer. Consequently, we suggest that the relationship between sika deer and Japanese serow is competitive in terms of diel activity patterns.