In the image data processing of the digital copying machine (M), the image data is assigned to the expanders (30,31) in such a manner that the expansion time of each of the expanders (30,31) is close to one another. Thus, because the difference of the expansion time between the expanders (30,31) is very close to one another, both of the expanders (30,31) can be efficiently used so that the timing when printing can be started is quickened. In consequence, the speed of expansion processing or print processing is improved.