A client device and a server device are connected to each other via a network. The client device sends an acquire request containing an identifier that indentifies the partial image data to the server device. The server device extracts the identifier from the acquire request, and acquires image information about the partial image data that corresponds to the identifier from an identifier table. Then the server acquires partial image data corresponding to the image information from a partial image data database and sends the acquired partial image data to the client device via the network.