In this study, we investigated the perceptual rigidity in the cross illusion with illusory rectangles ; then we found that the perceptual rigidity is changed according to the supporting length of rectangle and the interpolated span as an illusory surface. Namely, the distortion is more difficult to be perceived in the longer supporting part condition and easier to be perceived in the longer interpolated span condition. We can easy to understand that these perceptual properties as an analogy in real world with the beam characteristics in the strength of material. However, the rigid perception relating to the other parameters such as shapes, width and so forth, was not apparent in the present experiments.