Control for adding additional information to an input image is efficiently done in correspondence with the recording sheet used without any sensors for detecting the type of recording sheet. To achieve this object, according to this invention, whether or not the additional information is to be added to input image data to be hardly identified by the human eye is controlled in accordance with the type of image formation process. Alternatively, a method of adding the additional information to input image data to be hardly identified by the human eye is controlled in accordance with the image processing mode corresponding to the recording sheet.