Range shifts of mountain organisms toward higher elevations in response to global warming may result in spatial mismatches between plants and their pollinators. Here, we aimed to examine whether bumblebee diversity decreases in a high‐altitude zone, and whether it parallels a decrease in the altitudinal diversity of bumblebee‐visited plants. We surveyed the alpha diversities of flower‐visiting bumblebees and bumblebee‐visited plants along an altitudinal gradient on a Japanese high mountain. Then, we examined whether the alpha diversities of bumblebees and bumblebee‐visited plants could be explained by altitude, or by other factors such as season, surveyed area and flower abundance. We found that a model including only altitude best explained bumblebee diversity, and that flower abundance and plant diversity had considerable value in explaining bumblebee diversity. In contrast, none of the studied factors explained plant diversity. Bumblebee diversity was minimal in the high‐altitude zone (1,900–2,600 m a.s.l.), where the only dominant bumblebee species, Bombus beaticola, visited many species of flowering plants. In contrast, five to seven bumblebee species were distributed in the low‐ (700–1,300 m a.s.l.) and middle‐ (1,300–1,900 m a.s.l.) altitude zones. These results show that plant–pollinator mutualism in high‐altitude zone of a Japanese mountain is asymmetric: many bee‐pollinated plants rely almost exclusively on one bumblebee species (B. beaticola) for pollination. Monitoring future changes in the distribution and abundance of B. beaticola is indispensable for the conservation of alpine plant in Japan. We aimed to examine whether bumblebee diversity decreases in high–altitude zone, and whether it parallels altitudinal diversity decrease of bumblebee–visited plants. We show that the model including only altitude best explained bumblebee diversity, which decreases in high–altitude zone, and in contrast, no appropriate factors including altitude explained plant diversity. These results show that plant–pollinator mutualism in high–altitude zone of Japanese mountain is asymmetric: many bee‐pollinated plants rely almost exclusively on one bumblebee species (B. beaticola) for pollination there.