To correct the field angle of an image sensing device which senses an image of a real space that includes a feature point whose position on the real space is known, the position of the feature point (real marker position) in the image of the real space sensed by the device is detected. Then, the feature point position (virtual marker position), which is seen when the real space is viewed from a virtual viewpoint specified by the measured position, orientation, and field angle of the device, in that image is calculated. A correction amount of the position and/or a correction amount of the orientation of the device are/is calculated so as to minimize a deviation between the real and virtual marker positions in the image. The correction amount of the field angle of the device is calculated in accordance with the minimized deviation or the correction amount of the position.