A PDL interpreter rasterizes image data, which are described in page description language, entered from a host computer, and stores the rasterized image data in an image memory. A CPU discriminates the type of image, pixel by pixel, by referring to the image data stored in the image memory, and stores a discrimination code, which indicates the result of discrimination, in a code memory. The CPU reads out image data that have been stored in the image memory and reads the discrimination code corresponding to these image data out of the code memory. The image data read out of the image memory are subjected to different binarizing processing by two binarizing circuits. Depending upon the discrimination code read out of the code memory, a selector selects the output of one of the two binarizing circuits and sends the selected image data to a printer. Accordingly, it is possible to obtain a high-quality image which is a mixture of images having different characteristics, such as an image containing character or line drawings, continuous grayscale images, etc.