The characteristic appearances of geometrical illusions (GIs) were investigated under a "special environment" in which the GIs are exposed under continuous and intermittent presentations of a homogeneous red background (BG). Six typical GIs (Muller-Lyer, Ponzo, Delboeuf, Ebbinghaus, Hering, and Wundt) were used in two experiments. Six subjects were asked to report the appearances of the GIs under two kinds of BG presentation. The apparent depth of the GI segments alternated remarkably with the unstable localization in visual space. The general magnitudes of illusion (MIs) represented almost normal and stable estimations for most of the GIs. In contrast, the MIs in the GIs, particularly those constructed by cross segments varied clearly under the two kinds of BG presentation. These results suggest that the factor based on the stimulus configuration is influenced little through the BG manipulation, however, another factor based on the stimulus presenting condition is affected under the "special environment".