In an image forming apparatus for reading a document image and forming the document image on a paper, document image data are divided into a plurality of blocks and the document image data are compressed in the unit of block to be stored in a first memory device. The data read from the first memory device are expanded in the unit of block to be stored in a second memory device. Compression time and expansion time are estimated and image output start time for a printer is set based on the compression and expansion times before expansion of all the blocks is completed. In a different way, the start time is set to read data from a second memory device by a printer for printing, after estimation based on expansion time on data not yet expanded by an expansion device. In a still different way, predetermined form data such as page number are superposed with image data. The form data is drawn first in a page memory device and expanded image data are superposed next in the page memory device. Print is started by taking drawing time of the form data into account.