Recently, it has been reported that conventional probability law can be violated in the context dependent phenomena which are "adaptive" to the environment or experimental settings. Also, it has been discussed that non-Kolmogorov probability, e.g. quantum probability, is available for modeling such a context dependent phenomenon. In this study, we investigate context-dependency of human perception process for optical illusion of Schrader stair, and we propose the non-Kolmogorov probabilistic model for the illusion.