An image registration server stores encryption keys of respective ones of a plurality of client terminals. Image data is encrypted by the image registration server using the encryption key corresponding to the client terminal to which the image data is applied. The encrypted image data is applied to the client terminal via a server. The client terminal has a decryption key stored within so that only a client terminal that is duly authorized can decrypt encrypted image data. Other client terminals that are not duly authorized cannot decode the image data. This makes it possible to prevent unlawful use of image data even if the image data has been intercepted.