A previous quantitative analysis of the food composition of the Eurasian harvest mouse (Micromys minutus) in Japan showed that it is insectivorous and granivorous. This supports the expectation that such a small mammal requires highly nutritious foods. Other studies have analyzed the feces of harvest mice, but these were only collected during winter in order to minimize disturbance of the animals. In the present study, we collected samples from all four seasons in order to understand changes in diet throughout the year. Results showed apparent seasonal differences in the diet of harvest mice. Insects accounted for ca. 30% of the diet in summer and autumn and seeds increased from 27% in winter to 50% in spring, suggesting the insectivorous nature of the harvest mouse in summer and autumn and graminivorous nature in winter and spring. These results strongly suggest that the harvest mouse is an opportunistic feeder. It has previously been thought that the harvest mice capture insects in the stalk zone of tall grassland community, but here, DNA analysis shows that harvest mice feed on ground-dwelling invertebrates, such as pill bugs (Armadillidium sp.) and carrion beetles (Calosilpha sp. or Ptomascopus sp.). This suggests that the harvest mouse goes down to the ground to feed on them.