A conversion server and a plurality of clients are connected via a transmission line. The conversion server that receives image coded data from the clients converts the image coding system in accordance with the coding system available to each client, the coding setting, and the status of the transmission line, and transmits the converted image coded data. For a client where the number of images that can be displayed is limited, the conversion server decodes a selected plurality items of image coded data, re-encodes the selected pieces into one composite image, and transmits the re-encoded image. The conversion server also comprises decoding processing units, one for each connected client, for decoding image coded data from each client. In response to a display image switching request from a client, the conversion server intraframe-codes the decoded image data of an image that will be used after the switching and transmits the intraframe-coded data. This enables an image to be switched quickly independently of the intraframe time of received image coded data that will be used after the switching.