Fraxinus lanuginosa Koidz. is a tree species distributed in Japan and in southern Korea. In central Hokkaido, northern Japan, hermaphrodites and males commonly coexist in populations of the species. Hermaphrodites and males have identical flowering phenologies and pollen morphologies. However, the mean germination rate for pollen from hermaphrodites, on agar culture media, is 2.8–3.4 times lower than that for pollen from males. The mean seed set of inflorescences cross‐pollinated by pollen from hermaphrodites is similarly 2.7 times lower than that by pollen from males. In contrast, the mean seed set of hermaphrodite‐pollinated inflorescences is 12–19 times more than that of self‐pollinated inflorescences. These results indicate that F. lanuginosa is functionally androdioecious and exhibits strong self‐sterility. The findings help explain the maintenance of androdioecy in F. lanuginosa.