1. An image display system for providing first and second images such that a right eye and a left eye of an observer see the first and second images at different times, respectively, the image display system comprising: image sensing means for sensing an image of the observer; a spatial modulation device transparent to light, for displaying the first and second images at the different times, respectively; generation means for generating a pair of back light images based on an image output signal from the image sensing means at the different times, respectively; a back-lighting device which receives an output signal associated with the back light image generated by the back light image generation means and displays the back light images to illuminate the spatial modulation device from a back side thereof, at the different times; synchronization means for synchronizing one of the first and second images present on the spatial modulation device with a generation of one of the pair of back light images by the generation means; and an optical element for giving directivity to back light emitted by the back-lighting device so that the back-lighting device provides back light illuminations of the spatial modulation device for the right and left eyes at the different times, wherein the generation means comprises: image processing means for processing an image of the observer's whole face sensed by the image sensing means, detecting a centroid of the face area of the image of the observer, and then generating the pair of back light images on the basis of the detected centroid.