Recently, the range of the deer has expanded worldwide, with dramatic increases in abundance; yet relatively little is known about how these increases influence other mammals at high trophic levels. A recent study showed that high deer density exerts cascading effects on raccoon dogs, omnivorous carnivores, because of any increase in their prey abundance in Oku-Nikko, Japan. Here, we examined the diet of Japanese badgers, another omnivorous carnivore, outside a deer-proof fence and compared carnivore abundance inside and outside the fence in Oku-Nikko, to assess whether high deer density outside the fence influences the carnivore. Earthworms and insects, the population of which increased by high deer density, were frequently found in badger fecal samples outside the fence. Outside the fence, the deer density was 13.5/km² and the badger relative density was 0.16, whereas inside the fence, the deer density was only 2.3/km² and the badger relative density was 0.01. A statistical test showed that this inverse relationship was significant. We propose that the grazing pressure associated with high deer density impacts habitat conditions, increasing earthworm and insect abundance, which then increases omnivorous carnivore foraging opportunities and population densities.