An image scanner using an area sensor having a tilt reads a plurality of low-resolution image data having phase shifts from each other, and the low-resolution image data are converted into those on an orthogonal coordinate system by affine transformation. The number of data to be used is decided based on one of these low-resolution image data. The low-resolution image data as many as the designated number of data are saved, and high-resolution image data is generated by executing super-resolution processing.