Print data including text or graphics data is transferred from a host computer to a printer. The printer develops the received data on band basis. When data processing to be performed by the printer is expected to fail due to shortage of the memory area size, the host computer cancels the data which has already been sent to the printer, and develops the text or graphics data to form image data. The image data thus formed is sent to the printer. Image data is transferred in a hierarchical manner. Disclosed also is a method which makes it possible to eliminate as much as possible any data processing failure in the printer attributable to shortage of memory capacity.