Adult burying beetles use the carcasses of small vertebrates for food and breeding their young. Some species are known to have optimum carcass sizes related to their body sizes, but their carcass use is difficult to observe in situ. We explored their size-related niche differentiation in carcass resource using stable isotope analysis. The largest Nicrophorus species in Japan (N. concolor) showed the highest δ15N, and the smallest Nicrophorus species in Japan (N. montivagus) showed the lowest δ13C and δ15N among all Nicrophorus species, suggesting that their niches are differentiated from those of other species in carcass resource. We also revealed a positive correlation between δ15N and body size. Ptomascopus morio showed significantly higher δ15N than all studied Nicrophorus species. This species does not show parental care behavior and the adults may feed on quite different resources from Nicrophorus species.