1. A computer-readable storage medium having stored therein an image display program for stereoscopically displaying a real world image synthesized with a three-dimensional virtual object on a screen of a stereoscopically-viewable stereoscopic display member by using outputs from a right-eye real camera and a left-eye real camera, wherein: the image display program instructs the computer to function as: a first position/orientation calculation section for recognizing a predetermined image-capture object in right captured image data outputted from the right-eye real camera so as to calculate first position/orientation information representing a relative position and orientation between the right-eye real camera and the predetermined image-capture object in a predetermined coordinate system; a second position/orientation calculation section for recognizing a predetermined image-capture object in left captured image data outputted from the left-eye real camera so as to calculate second position/orientation information representing a relative position and orientation between the left-eye real camera and the predetermined image-capture object in a predetermined coordinate system; a virtual camera setting section for determining a position and an orientation for both of a right virtual camera for producing a right-eye image and a left virtual camera for producing a left-eye image in a predetermined virtual space by using at least one of the first position/orientation information calculated by the first position/orientation calculation section and the second position/orientation information calculated by the second position/orientation calculation section; a right virtual image production section for producing a right virtual image showing the virtual space as seen from the right virtual camera; a left virtual image production section for producing a left virtual image showing the virtual space as seen from the left virtual camera; a display control section for synthesizing the right virtual image with the right captured image and synthesizing the left virtual image with the left captured image so as to output an image for stereoscopic viewing to the stereoscopic display member; a camera interval calculation section for calculating a distance in the predetermined coordinate system between the right-eye real camera and the left-eye real camera by using the first position/orientation information calculated by the first position/orientation calculation section and the second position/orientation information calculated by the second position/orientation calculation section; and a correspondence information calculation section for calculating correspondence information between a unit length in the predetermined coordinate system and a unit length in a real space by using the distance in the predetermined coordinate system calculated by the camera interval calculation section and a distance in a real world between the right-eye real camera and the left-eye real camera.