An image processing unit includes an image data input unit, a page-description language (PDL) converter, a special document determination unit, and an image treatment command adding unit. The image data input unit inputs image data. The PDL converter converts the input image data to page-description language data, and transmits an end-of-page command when a page-description language conversion for image data is completed. The special document determination unit determines whether the input image data includes a special document. The image treatment command adding unit instructs the PDL converter to wait for a transmission of the end-of-page command until the special document determination unit issues a determination result. When it is determined that the image data includes a special document, the image treatment command adding unit generates an image treatment command for the image data. The image treatment command adding unit transmits the image treatment command with the end-of-page command.