A compound image-forming method and compound image-forming apparatus capable of handling image formation without prolonging processing time even if the compression system is not able to carry out partial decoding of a compressed image. The present invention reads a document, partitions a one-page image into an arbitrary number of units at the same time while the document is being read, and compresses the partitioned image, thereby making it possible to use a common image, and, in a copying operation, enabling printing without waiting for an entire page to be read, thus making it possible to enhance printing performance capabilities for fast printing and others. Further, in a facsimile transmission, transmission can be carried out without waiting for an entire page to be read, thereby enabling communication time to be reduced.