Reproduction carries costs, in terms of growth, survival, and future reproduction, because reproduction entails the expenditure of energy and time that could be used for other functions. Male <i>Pagurus</i> hermit crabs engage in precopulatory mate guarding behavior for a period of up to several days, and this guarding behavior may entail costs for males. We investigated whether precopulatory mate guarding depresses feeding rate and molting frequency as a cost of reproduction for males in the hermit crab <i>Pagurus nigrofascia</i>. Males were discriminated into the following three size classes: small males that were not observed to guard females, middle-sized males, and large males that guarded females. In the field, molting frequency of the large males was significantly lower than that of the other two size classes. The feeding activity of males was greatly depressed while they were engaged in guarding behavior. The duration of guarding was affected by sex ratio in laboratory experiments, the total guarding time increasing along with the number of mates. Molting frequency did not differ significantly among male crabs under the different sex ratios, however. We suggest that precopulatory mate guarding may entail a reproductive cost for males of <i>P. nigrofascia</i> due to depression of feeding activity although this cost may be compensated for by higher feeding activity before and/or after guarding.