To avoid predation, many prey animals remain motionless, which enhances crypsis. It is generally considered that once predator detects the immobile prey, the motionless state loses the function of crypsis, and thus the prey should not remain motionless because allowing predators come close simply makes prey difficult to escape. Contrary to this expectation, frogs often remain motionless even when they are detected by predator snakes, and the frogs initiate fleeing at a closer distance. To reveal possible advantages of the response, we conducted field observations to comprehend the situation of predation event of them. According to the observed situation, we conducted staged encounter experiments using frogs and snakes to examine factors that affect successful escape of frogs, especially focusing on strike behavior of snakes in close quarters. From the field observations, it was confirmed that the predation events between frogs and snakes are settled within a few seconds because of much safe zone for frogs around the predation event. In the experiment, video analysis revealed that snakes are not able to change the course of strike after initiating it, and not able to move in a split second after the strike behavior. Thus, it was demonstrated that frogs are able to evade the snake strike with a certain distance, and letting snakes to initiate strike and evading it is effective to make snakes pause in a split second that allows frogs to escape successfully. In conclusion, flight initiation of frogs after initiation of snake strike may increase the probability of successful escape, and thus, waiting for strike of snakes until they closely approach can be an adaptive decision that results in head start. This mechanism may explain the immobile response of frogs under being detected by snakes.