Spontaneous EEGs were measured at 12 electrode positions over the entire head when subjects perceived alternately one of two stimuli, that were given separately to the left and right eyes, in binocular rivalry. Upward and downward triangles of the same size were used as the two stimuli. The EEGs when subjects perceived a fused image in binocular fusion by receiving the same stimuli, i.e. upward or downward triangles, to the left and right eyes, under the same experimental conditions to binocular rivalry, were also measured for comparison. The difference of the EEG spectral powers obtained under the two states of binocular rivalry and binocular fusion was analyzed in the theta (4-7 Hz), alpha (8-13 Hz) and beta (16-24 Hz) bands. A significant difference in the power was obtained in the alpha band, although the physical parameters of the stimuli, i.e. the shape, luminance, size and contrast, given to both eyes in the states of rivalry and fusion were identical. A large difference of the alpha power was observed at the occipital, parietal and posterior temporal regions.