The rumen contents were collected from 36 wild Yeso sika deer (Cervus nippon yesoensis) captured by deer culling or by hunting in the spring, summer, autumn and winter in Hokkaido, Japan. Botanical classification was conducted, and the contents of mineral (calcium (Ca), phosphorus (P), potassium (K), sodium (Na), iron (Fe), copper (Cu) and zinc (Zn)) were measured. The animals were captured around pastures or fallow field areas in the Kushiro area. The rumen contents consisted of grasses and Sasa sp. leaves regardless of the season. Leaves and bark were ingested in the spring, autumn and winter. The macro-mineral contents in the rumen showed seasonal changes. In the summer, the Ca, K and P contents were high, and the Na content was low. There were no seasonal changes in the Fe content. The P, Na and Fe contents were higher than the animals' requirements. In a future survey, it is needed to determine the mineral contents of the food ingested by wild Yeso sika deer.