We examined the diet of Hyla japonica, a prolonged breeder, in the reproductive season (early May to late July). This species breeds in rice fields. Females attended the breeding site only at the time of spawning, but males remained and foraged in rice fields for their continuous advertising during the prolonged season. A high frequency of empty stomachs and fewer and less stomach contents observed at the beginning of the breeding season, may result from energetic constraints associated with reproduction in males. Diverse ground-dwelling invertebrates predominated in the diet, but aquatic organisms were quite few. We estimated the prey availability in the environment by sweeps. The most easily available prey such as ants, beetles, dipterans, caterpillars, and spiders were numerously consumed by frogs. Significant correlations between the diet compositions and prey availability suggest that H. japonica is an opportunistic predator.