The dotillid crab Ilyoplax pusilla performs waving displays during the reproductive season. When a female approaches a male, he often directs his waving at her. More often, however, males do not direct their waves at any particular individual. We conducted a field experiment that manipulated neighbors' sex to clarify whether either of the sexes was more often the target of waving displays. Waving frequency by males with male neighbors was significantly higher than that of males with female neighbors. Waving frequency was correlated with the number of active male neighbors but not with the number of active female neighbors. These findings suggest that waving display of I. pusilla is performed through male to male competition.