We investigated the current distribution of the sika deer population and its impact on vegetation around Mt. Hyonosen, Hyogo Prefecture. Based on a vegetation survey that was conducted at a regional scale, we estimated that understory vegetation in deciduous hardwood forests has declined significantly over mountain ranges on the east- and south-sides of Mt. Hyonosen. A pellet group count survey indicated that there has been an overabundance of sika deer since 1999 in these areas. It is hypothesized that a neighboring population of sika deer with high density expanded into these mountain ranges. Moreover, recent decreases in snow in this region could have promoted population expansion. Vegetation around the peak has not declined yet, but deer grazing has occurred seasonally due to migratory individuals traveling from the foot of the mountain. Signs of deer grazing were observed on 230 plant species, including 13 red-listed species, on Mt. Hyonosen. Moreover, we found that a community of red-listed species had declined because of deer grazing. We note that serious declines in vegetation could expand to the peak of Mt. Hyonosen if management of the deer population is not rapidly introduced.