To provide a printer dealing with capacity insufficiency of a memory without deteriorating a quality level of print data.

A printer is connected to a host computer 101, and has a ROM 103, and a RAM 114. The ROM 103 has a lithographic memory forming unit 105, a rasterizing processor 106, a packed bits compression processor 108, and an ED method compression processor 108. Image data received from the computer 101 is rasterized to form a lithographic object. The object is packed bits compression-processed to reduce its size and to store it in a lithographic memory 115. If memory capacity insufficiency of the memory 115 occurs, its extent is checked. If the insufficiency is large, resolutions of both main scanning and sub-scanning are lowered, and Rossi compression-processed. If it is intermediate, the resolution of either the main or sub-scanning is lowered, and it is Rossi compression-processed. If it is small, gradation is lowered from 2 bits to 1 bit, and it is Rossi compression-processed.