[Problem]To provide an image data distribution server that is capable of distributing digitally watermarked image data that correspond to the ranks of user access rights to client terminals.[Solution]An image data distribution server (11) has: an image data generation means that generates multiple sets of image data of various kinds on individual images of multiple pieces of electronic information of various kinds that are stored in the server; a rank-based classification means that, upon receiving requests from users for distribution of the image data, classifies the access rights of the respective users to the image data in terms of ranks on the basis of authentication results for the users; a display form changing means that adds digital watermarks that correspond to the respective access right ranks classified by the rank-based classification means to the image data in an unremovable manner; and an image data distribution means that distributes the multiple sets of image data of various kinds, including the image data to which the digital watermarks are added, to client terminals (12A-12C).