Abstract: An image reading apparatus having a line image sensor for line scanning a document is disclosed. The line image sensor is comprised of plural sensor chips aligned in the main scan direction thereof each of which has an output terminal for outputting each image signal obtained thereby. The image reading apparatus provides plural analogue to digital conversion means for converting respective analogue image signals outputted from respective image sensor chips into digital data, respectively, plural memory means for memorizing the digital data and an output means for reading said digital data from said plural memory means parallel and for outputting those digital data having been read serially.