Information capable of tracing a content distribution route, such as a user ID of a content destination, a content ID, a content provider ID, and a service provider ID, is superimposed on the content as additional information for digital watermarking or steganography before delivering the content. For example, the content ID and the user ID are detected from the delivered content. When both the content ID and the user ID are not detected from the content, it is determined that the content is illegally distributed, and the content distribution route is traced to identify the source of the illegally distributed content. In most cases, unique information, such as a user ID, still remains in copies of the content, thus effectively preventing illegal distribution of the content.