When image information for the purpose of being printed out has been entered, whether or not the image information contains a mark image such as a watermark is determined to such an extent that will not lower the throughput of a printer. To accomplish this, the image is input and elapsed time starts being measured from the start of detection processing that is for detecting whether a specific image is contained in the input image. If time runs out during the course of detection processing, a decision is rendered to the effect that the image does not contain the specific image.