The present invention enables to increase the speed of recognizing characters on an undefined-size postal matter such as a flat mail. The flat mail often has an address area different in color or pattern from the flat mail itself. The present invention provides an image input apparatus including pickup means of monochromatic-mode CCD 31 having a fine resolution capable of recognizing characters and pickup means of color-mode CCD 30 having a rough resolution only sufficient for detecting an address area. These pickup means are mounted in a unitary block so that images are taken in from approximately identical positions and a color image is used to detect an address area and a monochromatic image is used to recognize characters, thereby significantly increasing the speed of flat mail classification. An illumination unit 44 is forcibly cooled so as to enable to use a large-output lamp tube 75 capable of illuminating a wide range to assure the character recognition.