Canopy subsidies have been attracting attention as food resources sustaining high-density deer populations in heavily browsed habitats; however, evaluation of the contribution of canopy subsidies to deer diets is still limited. We investigated the seasonal variation in and the nutritional value of the diet composition of a sika deer population in the Tanzawa Mountains using 126 rumen content samples collected from May 2015 to June 2016. The deer population depended more on evergreen tree leaves in the winter than it did in the other seasons. In the summer and autumn, the occupancy ratio of woody tissues (e.g., twigs) was relatively high compared with other seasons. Even in the spring, woody tissues and deciduous tree leaves accounted for a certain proportion of the sika deer diet. The crude protein (CP) content of the rumen content samples collected in spring (9.56–22.69%) satisfied the minimum dietary CP requirement estimates for maximum growth. The CP content of the rumen content samples collected in the other three seasons satisfied the minimum dietary CP requirement for maintenance. We suggested that canopy subsidies make high quantitative and qualitative contributions to the sustainment of high-density deer populations in heavily browsed habitats.