This paper analyzes the expansion of sika deer (Cervus nippon) wintering ground in relation to global warming in Nikko, central Honshu, Japan. The sika wintering ground of 1993-1994 expanded toward snowy areas compared to that of 1980-1982, and the sika density increased both in new and traditional wintering grounds. Information on snow accumulation and winter deer distribution was collected by interviewing seven local inhabitants; they stated that snow accumulation decreased from more than 100 cm before the 1970s to less than 30 cm by the 1980s in Oku Nikko, and that sika deer were found to winter there since about 1985. Long-term warming trends and the shortening of the winter season were shown by records of air temperature and snowfall between December and March at the two meteorological stations in and out of the study area, the Nikko Weather Station (1944-1994) and the Utsunomiya Local Meteorological Observatory (1891-1994). The increase of average air temperature of December was obvious. It is concluded that recent snow decrease in Nikko is most likely a result of global warming, and that this probably has enabled sika deer to expand their wintering ranges toward snowy areas.