1. An image reading apparatus comprising: an image reader which detects a light reflected from a document and sends gradation data on the reflected light as digital data for each pixel of an image of said document; a histogram generator for counting frequencies, at each gradation level of the digital data received from said image reader, to generate a histogram; a detector for detecting whether the image of said document is a bi-level image, a map image, or a magazine image according to the frequencies in the thus generated histogram; and a data converter for converting said gradation data according to a detecting result of said detector.