A double-side digital copier reads an image on documents fed into an image reading unit in accordance with a predetermined cycle based on copying conditions such as the number of copies to be made and single/double-side copying, and the number of documents and copy sheets temporarily maintained in the respective circulation paths formed therein. Further the image data obtained by that reading are stored in two memories alternately, one of the memories alone, or both the memories in a predetermined order in accordance with the predetermined cycle, and are read out in a corresponding order, optimizing throughput without increasing the manufacturing costs.