Flowering phenology may affect flower visitor community dynamics. In the mountainous region, flowering phenology differs between low and high altitudes. In high mountains, flower resources are seasonally limited (June to September) compared to lower altitudes (April to November). For this reason, the community structure of bumblebees which are main pollinators in high mountains may change greatly along altitudinal gradient. Here, we introduce an example of field observations carried out at Mt. Norikura, central Japan. First, community structure of bumblebees greatly changed along altitude. The diversity of bumblebees was high (5-7 bumblebee species) at low (700-1300 m a.s.l.) and middle (1300-1900 m) altitudinal zones, while only a single, small bumblebee species (Bombus beaticola) dominated at high altitude (1900-2600 m). In contrast, the diversity of the bumblebee-pollinated plants did not decrease at high altitude, so that the small B. beaticola visited many species of flowering plants there. Next, we show that the flower sizes of two widely distributed plants (Prunella vulgaris and Campanula punctata) correlate with average body size of local bumblebees: they were small at high altitude where small B. beaticola dominate. These results suggest that climate changes in mountains may affect flowering phenology and may further influence the community structure of bumblebees and flower size of bumblebee-pollinated plant.