A position of an observer in a stereoscopic observing region is detected by a position detecting unit. A right-eye image and a left-eye image which are seen from the detecting position are formed by an image forming unit and displayed on a display. By setting an aperture position of a projection optical system, the right-eye image is projected to the right-eye position of the observer and the left-eye image is projected to the left-eye position, thereby allowing a stereoscopic image to be observed. Further, an aperture is set so as to project the right-eye image or left-eye image to a position different from the detecting position of the observer, thereby allowing a same image to be seen to both eyes of another observer and allowing a two-dimensional image to be observed.