1. A three-dimensional shape measuring apparatus which measures a three-dimensional shape of an object, said three-dimensional shape measuring apparatus comprising: a mirror which reflects light from the object; an image capturing unit configured to capture an image including both a real image and a virtual image of the object, the virtual image being formed using said mirror; a light amount changing unit configured to change a light amount of one of the real image and the virtual image; an image separating unit configured to separate the real image and the virtual image in the image captured by said image capturing unit; and a reconstruction unit configured to reconstruct the three-dimensional shape of the object, using the real image and the virtual image that are separated by said image separating unit, wherein said image capturing unit is configured to capture a first image which is the image influenced by the change caused by said light amount changing unit, and a second image which is the image uninfluenced by the change caused by said light amount changing unit, and based on a comparison between the first image and the second image, said image separating unit is configured to specify, as the one of the real image and the virtual image, a portion having a difference in light amount greater than or equal to a predetermined threshold between the first image and the second image, and specify, as the other of the real image and the virtual image, a portion having a difference in light amount smaller than the predetermined threshold between the first image and the second image, to separate the real image and the virtual image in the image.