An available image area is recognized from information, including a read start position and a read end position of a document, a scaling for performing image processing, a processing object position, or the like. Subsequently, the number of chromatic pixels of one line is counted to output it to a memory, and when it is summed up to an end line of the available image area, a difference between a count number N1 in one previous line of the available image area and a count number N2 in the end line of the available image area is calculated to thereby compare the difference with a predetermined threshold value. When the difference is larger than the threshold, it is determined to be a chromatic image, whereas when the difference is smaller than the threshold, it is determined to be an achromatic image.