A CPU perform the steps of: (a) causing a compression/decompression processor to decompress the compressed data of one of three bands in the data area except for the first block in the band, and storing decompressed bitmap data in the data area; (b) rasterizing each of the intermediate data blocks in the band and synthesizing the rasterized data and the decompressed bitmap data in the band; and (c) causing the compression/decompression processor to compress the synthesized bitmap data and storing the compressed data in the data area. The CPU performs the steps (a) to (c) in different respective tasks in parallel, and performs the steps (a) to (c) along the order of (a), (b), (c) for each of the intermediate code blocks in each of the bands while using the 1st to the 3rd bitmap data area in turn for each of the steps (a) to (c).