An authentication server: has user identification information and a password of that user identification information previously stored therein in association with each other; receives user identification information transmitted, the user identification information being determined from information input at and received by an image output apparatus; obtains a password received by an image transmission apparatus used by a user associated with the received user identification information, from that image transmission apparatus after that user identification information is received; determines whether the obtained password and the password stored in the authentication server in association with the received user identification information match; and, if a decision is made that the passwords match, permits the image output apparatus that had sent the received user identification information, to output an image corresponding to image data transmitted from the image transmission apparatus and stored in that image output apparatus in association with that user identification information.