In order to prevent illegal copying more effectively in an image forming apparatus, when a specified ground pattern for preventing copying is detected in input image data, the detected ground pattern is emphasized in the image data. Alternatively, information for detecting the specified ground pattern is stored in a storage device, in order to detect the ground pattern with the stored information. When the ground pattern is not detected, a ground region in the image data is analyzed further to determine whether a different ground pattern exits or not. When a ground pattern is detected, information for detecting the ground pattern is stored in the storing device.