The image reading device comprises an imaging device on which an on-chip color filter is provided. The on-chip color filter has color filter elements of the Bayer arrangement, for example. Pixel data, output from the imaging device, are stored in a memory. When the pixel data are read from the memory, some of the pixel data are thinned out, so that a thinned image data, colors of which are arranged in the same arrangement as the color filter elements of the imaging device, is obtained. The pixel data of the thinned image data are subjected to an interpolation process, so that one image's worth of pixel data are generated for each of the color components.