For the conservation of the diving beetle Dytiscus sharpi (Wehncke) (Coleoptera: Dytiscidae), which is included on the Red Data List of Japan, it is critical to understand its ecological background. In the present study, we focused on its feeding behavior and nutritional needs under laboratory breeding conditions. First, we made a list of the possible candidates of prey in the same habitats where we caught D. sharpi. We found that the tadpoles of Rana ornativentris (Werner) were the major species present from March to April, when the beetle larvae appeared. Second, under our laboratory conditions, we investigated the size preference of beetle larvae preying on R. ornativentris tadpoles. We found a significant positive correlation between the developing stage of the larvae and the preferred prey size, i.e., the first and third instars preferred smaller and larger prey, respectively, but second instars did not show any size preference. The size of full–grown adult beetles was almost the same as that of wild insects found in the field, indicating that R. ornativentris tadpoles provide almost complete nutrition for larval growth. Finally, we investigated how the size and number of R. ornativentris tadpoles were correlated with the developing stage of beetle larvae. We suggest that it is crucial for Dytiscus larvae to have access to tadpoles of the proper size and amounts, depending on their growth stage.