When image data is outputted in a special image processing apparatus which has printed confidential data before, a server confirms whether the user who has instructed output is a permitted user or an unpermitted user. If the user is a permitted user, the server permits printing. If the user is an unpermitted user, the server confirms whether a predetermined period has elapsed after the special image processing apparatus was used last. If the predetermined period has elapsed, the server permits printing. If the predetermined period has not elapsed, the server restricts output by stopping printing or by document filing for storing image data. Thus, by restricting use by an outside user, leakage of confidential data is prevented.