The invention provides an image data communication system of a simple construction wherein wasteful processing is eliminated so that desired image data can be transmitted rapidly to a terminal of a source of a request for the image data. A server and a plurality of terminals are interconnected so as to allow communication of image data. A terminal information retrieval section provided in the server discriminates whether or not image data requested from a first terminal have already been transmitted to a second terminal. An image data transfer section provided in the server requests, when a result of the discrimination is that the data have been transmitted already, the second terminal for transmission of the image data and transfers the image data received from the second terminal to the first terminal. A terminal memory provided in each terminal stores image data, and an image data transmission section provided in each terminal reads out image data requested from the server from the terminal memory and transmits the read out image data to the server.