An image scanner is provided with a super-resolution processing function for generating a high-resolution image using multiple pieces of low-resolution image data. The scanner device then measures an initial MTF value without performing the super-resolution processing, refers to an MTF properties table in which is registered a number of pieces of low-resolution image data for a scanner device of that type to achieve a pre-set target MTF, and registers the number of images that corresponds to the initial MTF value in the scanner, if that number is present, and registers an interpolated value if that number is not present. The image scanner then performs the super-resolution processing using the registered number of pieces of low-resolution image data, and outputs image data of a target resolution.