An image reading apparatus has an image sensor in which a plurality of sensor arrays for reading a document image are connected into a line, and parallel-output image data. In the image reading apparatus, of image data output from each sensor array, image data of a pixel at one end and image data of the remaining pixels are compressed by different compression methods. The compressed data of one line are decompressed by a set method at the timing when the number of pixels reaches a preset number of pixels.