We examined whether the presence and developmental stage of conspecific egg masses affected the choice of oviposition site by the Japanese brown frog, Rana japonica. A paired choice experiment was conducted in nine field enclosures, each containing one empty artificial pool and one containing conspecific egg masses at different developmental stages. Adult R. japonica almost always selected the tub with the resident egg mass for their oviposition site if the resident egg mass was younger than Gosner's stage 16, but avoided tubs that contained older egg masses. We hypothesized that the potential of cannibalism from conspecific residents increases as the resident eggs develop, which affects the choice of oviposition site by adult R. japonica.