Assessing the pollinator importance of each floral visitor to a plant species is a key to understanding plant-pollinator interaction. The present study examined visitation frequency, pollination efficiency, and pollinator importance of the full range of floral visitors to Geranium thunbergii natural population, by measuring seed-set. During 2 years of observations, the flowers were visited by at least 45 insect species belonging to four orders. Among the main 22 visitor species, 11 species belonging to three orders (Hymenoptera, Diptera, and Lepidoptera) acted as the efficient pollinators. In both years, Hymenoptera, especially bees, was the most important pollinator to G. thunbergii. Thus, the flowers could be considered bee-pollinated. However, the most important species were not constant between years. The study also documented that efficient pollinators have larger body sizes. The dish-shaped floral morphology, taxonomically diverse pollinators, and temporal change in the most important pollinators indicate that G. thunbergii–pollinator interaction is a rather generalized system. The results suggest that casual observations of visitation or even precise measurement of pollinator importance in a single season is insufficient to identify important pollinators.