R. rugosa are famous for giving out a smell when we catch them. They are rarely found in the natural diet of Japanese striped snake which is considered as a generalized predator. When we gave R. rugosa to the snakes, the snakes did not swallow them. It has not been clarified why R. rugosa are not eaten by the snakes. We hypothesize that the smell from their skin secretions has effects on anti-predator defense. We gave the snakes palatable R. japonica coated with the secretions of R. rugosa. The snakes took longer time to swallow the frogs than controls that we coated with the secretions of palatable frogs Fejervarya limnocharis. R. rugosa have effects with the avoidance of the predation by the snakes. To examine the effect of the smell to snakes, we gave the snakes the paper with the smelling secretions. Adult snake did not bite the paper with the smell of R. rugosa but bite the paper with the smell of palatable R. nigromaculata (control). On the other hand, newborn snakes behaved in the same way for the smell of R. rugosa and controls. The smell does not have effects for the newborn snakes that have not experiences biting R. rugosa. We suggest that the smell is a signal which tells unpalatability of R.rugosa for the snakes through experiences. In addition the smell reduced the activity of conspecific individuals of R. rugosa.