In a watermark process section in an image processing apparatus, a visible watermark pattern read-out section reads out a visible watermark pattern that is stored in a memory. An invisible watermark generating section draws an invisible watermark on the read-out visible watermark pattern. Image data, which includes both the visible watermark and drawn invisible watermark, is multiplied by an overlay ratio. An adder section adds the visible/invisible watermark image data, which is multiplied by the overlay ratio, and input image data. Thus, watermark-overlaid image data is generated.