The use of the undefined low-order four bits of an 8-bit watermark is defined and, when contents are reproduced, a predetermined operation is performed according to the information stored in the low-order four bits. That is, a watermark is added to an original image to create an image with the watermark inserted. When the contents are reproduced, the watermark is detected in the image and, based on the information stored in the low-order four bits of the detected watermark, a predetermined operation, for example, access to a web site on the Internet, is performed.