1. A stereoscopic image displaying method, wherein, when image information displayed on an image displaying device is observed three-dimensionally by guiding display light from an image corresponding to a viewpoint of one parallax image on said image displaying device, on which parallax images corresponding to a plurality of different viewpoints can be displayed, to an optical modulator, on which a light transmitting section and a light shielding section can be formed, by a second optical system disposed in the front of said image displaying device, and collecting the display light transmitted through said light transmitting section of said optical modulator at a position, which is a predetermined distance apart, corresponding to the viewpoint on an observation surface, by a first optical system, the entire screen of a parallax image to be displayed on said image displaying device is caused to be incident on each eye by controlling transmitted light from said optical modulator in synchronism with the switching of parallax images to be displayed on said image displaying device.