Using image data on a plurality of reduced object images to calculate a shift amount in regard to the gap of relative positions between the reduced object images by a correlation calculation between the reduced object images. A conversion equation is obtained from the shift amount for geometric projection process from the object image to each of the reduced object images. Initial image data on a single object image is generated using image data on the plurality of reduced object images. The above are used to estimate images of each of the reduced object images. A difference between the estimated image of the reduced object images and the reduced object images is projected in the reverse process of the geometric projection process, updating the image data on the single object image. The processes are repeated until the difference satisfies a predetermined condition and a high resolution object image is outputted.