Like European badgers, Japanese badgers are known to use earthworms as a staple food. However, while European badgers mainly eat the family Lumbricidae, Japanese badgers are thought to primarily take the family Megascolecidae in forest areas. In this study, we investigated the feeding habits and habitat utilization of Japanese badgers inhabiting a mountainous dairy farm, where a previously less-investigated mosaic of forest and open areas occurred. Invertebrates, particularly earthworms and beetles, formed the bulk of badger diet throughout the study period, while the frequency of occurrence of fruit and larvae partially depended on their seasonal availabilities. Soil sampling showed that both Megascolecidae and Lumbricidae were present in the study area, with the former being more abundant in natural forests and the latter being more abundant in pastures. Furthermore, their seasonal availability showed opposite patterns, with biomasses being the highest in summer for Megascolecidae and in spring and fall for Lumbricidae, resulting in the total biomass of earthworms in the whole study area remaining relatively consistent between seasons. Habitat use by Japanese badgers did not mirror food availability, pastures and natural forests being preferred throughout the year.