1. An image display system for superimposing a virtual space image onto a real world image and displaying the resultant composite image on an image display apparatus that is of a hand-held type, the image display apparatus including: a housing; a stereoscopic display screen which is provided on a front surface of the housing and is capable of providing an image that is stereoscopically visible by naked eyes; and a first camera and a second camera provided on a back surface of the housing, and the image display system that comprises a processing system that includes at least one hardware processor, the processing system configured to: automatically detect an object that is present in a real world image shot by at least one of the first camera and the second camera; determine and update positions and orientations of a pair of virtual cameras including a first virtual camera and a second virtual camera in a virtual space, based on detection of the object in the real world image; generate, based on the first virtual camera, a first virtual space image of the virtual space; generate, based on the second virtual camera, a second virtual space image of the virtual space; and a cause an image that is stereoscopically visible by naked eyes to be displayed on the stereoscopic display screen, the image based on a first composite image obtained by superimposing the first virtual space image onto a first real world image shot by the first camera, and on a second composite image obtained by superimposing the second virtual space image onto a second real world image shot by the second camera.