J. J. Gibson (1979/1986) proposed that animals perceive ‘affordances’, which are the functional utilities based on the properties of both the animals and the environment. If this is the case, animals should make judgements about what to do referring to the capability of their own action system. In this study, I examined a perceptual boundary between “stepping-over” and “passing-under” for two groups-the tall group and the short group. Subjects were individually requested to judge whether they would “stepover” or “passunder” a bar presented in front of them which was varied in height. I found that the mean bar-height to leg-length (B/L) ratio at the perceptual action-switching-point is invariant, or 1.07, for each group. This result suggests that animals perceive affordances in controlling action, which means that the environment for animals is structured as to various levels of action.