In the image data processing of the digital copying machine (M), number of blocks Bn of image data in auxiliary scanning direction is set to the number of expanders (30, 31) Dn times a natural number N (N.times.Dn). This configuration makes it possible to utilize the (30, 31) expanders efficiently regardless of whether the image is to be rotated by 90.degree. or not when expanding compressed image data and printing the image. Thus such a situation that the expanders (30, 31) are forced to suspend the expanding operation does not occur, making it possible to start printing earlier and improve the printing speed. In the image reader 1 of the digital copying machine (M), when dividing image data corresponding to original document into a plurality of blocks and compressing the image data in the unit of block, length of the original document in the auxiliary scanning direction is estimated from width of the original document (size in principal scanning direction). Then by setting the partition of divided blocks based on the estimated document length, the image data can be compressed without waiting for the end of image data input for each block, not doing over the compression process even when the end of the original document is detected, thereby maximizing the image data compressing speed.