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 section 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 imaging section and a second imaging section provided on a back surface of the housing, and the image display system comprising: a marker detection section which automatically detects a marker placed in a real world from a real world image shot by at least one of the first imaging section and the second imaging section; a virtual camera control section which determines and updates 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 the marker in the real world image detected by the marker detection section: a first virtual space image generation section which generates a first virtual space image by drawing the virtual space, based on the first virtual camera; a second virtual space image generation section which generates a second virtual space image by drawing the virtual space, based on the second virtual camera; and a display control section which displays an image that is stereoscopically visible by naked eyes on the stereoscopic display section, based on a first composite image obtained by superimposing the first virtual apace image onto a first real world image shot by the first imaging section, and on a second composite image obtained by superimposing the second virtual space image onto a second real world image shot by the second imaging section.