An image display system which improves the ease of use provided to an operator when an alteration is detected in image data that is to be displayed. An electronic watermark detecting section and forwarding request section are appended to a personal computer connected to a disk cache storing image data. The electronic watermark detecting section has a function of checking for alterations in the image data in the disk cache, and an interface to the forwarding request section is also appended thereto. A transmission section is appended to a personal computer connected to an optical disk device storing original image data. The forwarding request section has the function of sending a request from the electronic watermark detecting section to the transmission section, and the function of transferring the image data transmitted form the transmission section, to the electronic watermark detecting section. The transmission section has the function of obtaining corresponding original image data from the optical disk device, in response to requests from the forwarding request section, and transmitting same to the forwarding request section. This image display system is suitable for use in a document management system comprising an image registration terminal, a reading terminal, and a server.