In a heavy snowfall area in northeastern Japan, we clarified the diel activity patterns of and seasonal differences in three sympatric carnivores (red foxes, raccoon dogs, and Japanese martens) during winter and spring, and the interspecific differences among the activity patterns in each season. The activity time of the target species was obtained by camera-trap surveys at seven sites around the Experimental Forest of Yamagata University from January to June 2019. The diel activity patterns of each species, and seasonal and interspecific differences in the activity patterns were evaluated using the kernel density method. Red foxes showed a unimodal nocturnal pattern in spring, whereas they were also observed during the daytime in winter. Raccoon dogs showed a unimodal nocturnal pattern, and peak activity after sunset in both seasons. Japanese martens were clearly nocturnal during both seasons. Because there were a few interspecific differences in diel activity patterns among three species, species interactions had little effect on diel activity patterns. Our findings indicate that the diel activity of medium-sized carnivores responding to winter conditions may vary depending on species.