A digital copier using a laser card or like external storing medium for storing image data. A remaining amount of memory capacity of a laser card is displayed in terms of the number of papers of a particular size or by percent. Based on the remaining amount of capacity, image data are compressed and reduced, the reduction ratio being also recorded in the card. When the contents of a series of documents cannot be accommodated in a single card, the first card is provided with a mark which shows that the second card is to follow.