Digital content management system using electronic watermark

A system for managing a digital content, particularly a digital content to which a copyright is claimed, and a system for supplying a public- key which is used in the management of the digital content are provided. The digital content management program is embedded to an operating system of a user apparatus as a micro- kernel, a watch program or a watch command which is linked to the digital content management program is transmitted to the user apparatus by using a network or data broadcasting, and thereby, the illegitimate usage of the digital content is watched. A visible watermark is added to the digital content when illegitimately utilized, to restrain later usage. Even in regular usage, the route of copying or transferring the digital content can also be ascertained by adding an invisible watermark. Further, a public- key is put in a public- key distribution screen to be distributed by the network or broadcasting. Image data to which information on owner of the public-key or on the user is added as an invisible electronic watermark, is entered to the public-key distribution screen, so that the authenticity of the public- key and the user is checked by the electronic watermark.