1. A non-transitory 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 comprising instructions that instructs at least one processor to: recognize 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; recognize 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; determine 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 and the second position/orientation information; producing a right virtual image showing the virtual space as seen from the right virtual camera; producing a left virtual image showing the virtual space as seen from the left virtual camera; 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; calculating a distance in the predetermined coordinate system between the right-eye real camera and the left-eye real camera by using the calculated first position/orientation information and the calculated second position/orientation information; and calculating a ratio between a unit length in the predetermined coordinate system and a unit length in a real space by using the calculated distance in the predetermined coordinate system and a distance in a real world between the right-eye real camera and the left-eye real camera.