In field studies of the population ecology, knowing where fruits and seeds are being produced is often required. In dioecious species, production of fruits/seeds and pollen is spatially separated and knowing the sex of individual plants is not always feasible when some reproductive individuals are not flowering or fruiting. Identifying the sex of individuals thus requires long-term phenology data on an individual basis but this is rarely available. In this study, we tested the feasibility of using the potential difference in pollen concentration in soil underneath the crowns of individual plants to identify the sex of plant overhead in a dioecious species. We hypothesized that the pollen concentration in soil beneath male plants would be significantly higher than beneath female plants because only males produce pollen and pollen deposition should accumulate in the soil underneath the male plants over time. We collected samples from surface soil under both sexes of an insect-pollinated dioecious shrub, Aucuba japonica (Garryaceae). Pollen grains were present in surface soil in both Oe and A horizons, and pollen concentration under males was significantly higher than under females. Pollen concentrations beneath males were positively correlated with male plant height, potentially reflecting greater pollen production by larger individuals. Considering the small plant size and relatively low pollen production of A. japonica, the method may hold promise for studying other species, especially in the tropics. Knowing the sex of individuals may be facilitated by our time-insensitive, relatively low-cost method using soil pollen beneath individual plants.