A browser 21 of a mark user client 3 obtains Web page 11 from a mark provider server 2 and displays it. Then control is transferred to a mark reference program 22 when a mark is detected, and the program extracts digital watermark information from a mark image. This digital watermark information comprises referred data, and an action definition that includes an action class and an index of the referred data as a parameter. The mark reference program 22 refers to this action definition, refers to required data through the index included in the action definition, and then performs processing defined by the action class.