Some prey animals can escape from the digestive systems of predators after being swallowed. To clarify the ecological factors that determine the success of such an escape, we investigated how the bombardier beetle Pheropsophus jessoensis escapes from two toad species, Bufo japonicus and B. torrenticola, under laboratory conditions. Pheropsophus jessoensis ejects a hot chemical spray from the tip of the abdomen when it is attacked. Although all toads swallowed the bombardier beetles, 43% of the toads vomited the beetles 12–107 min after swallowing them. All the vomited beetles were still alive and active. Our experiment showed that P. jessoensis ejected hot chemicals inside the toads, thereby forcing the toads to vomit. Large beetles escaped more frequently than small beetles, and small toads vomited the beetles more frequently than large toads. Our results demonstrate the importance of the prey–predator size relationship in the successful escape of prey from inside a predator.