An analyzer estimates the time for rendering image data based on the number, types, etc., of rendering commands, stored in an intermediate memory, and determines whether or not there is a possibility of overrun. If there is the possibility of overrun, image data of respective bands formed in a band buffer are sequentially compressed and stored into a compressed image memory. When the image data for one page has been stored, the data in the compressed image memory is sequentially expanded and supplied to a printer engine. This avoids failure of image output due to overrun, while reducing the capacity of the image memory.