All threshold data applied to processing is read into a register so that threshold data read from threshold matrix memory can be recycled until the end of a scanning line for processing, and is selectively output to a plurality of comparison means for executing parallel comparison processing. The threshold data set in the register is shifted in order for repetitive use. The threshold data of the next scanning line is read into the register during the comparison processing, and upon completion of processing of one scanning line, comparison processing on the next scanning line is executed as pipeline processing. Threshold data read from the memory needs to be executed only once for each scanning line for processing, and threshold data can also be read during halftone data generation of the preceding line.