In the interocular suppression a stimulus presented in one eye is suppressed and a different stimulus in the other eye is perceived for a certain time interval. For strabismic observers the suppression continuously occurs between different retinal images made by the misalignment of eye positions. It was thought, in previous researches, that strabismus suppression was different from binocular rivalry suppression for normal observers. In this research the monocular contrast sensitivity was measured in suppression phase of binocular rivalry in order to investigate the mechanism of interocular suppression. It was obtained, in the 1 degree stimulus condition, that contrast sensitivity was higher in the fusion condition than in the suppression condition for normal observers. On the other hand, for a strabismic observer, contrast sensitivity dropped both in the fusion and in the suppression conditions. In the 15 degree stimulus condition, we obtained a similar tendency in contrast sensitivities between normal and strabismic observers.