To prevent an environment, where authentication information is decoded by means of a retrial, from getting when authentication is rejected by illegal authentication information.

When secret information output instruction is inputted from an operation display part of an image forming apparatus now, the image forming apparatus starts the operation (step S1), and a user inputs authentication information in accordance with a request of the image forming apparatus (S2). A system control part confirms the inputted authentication information (S3) and determines whether the authentication fails or not by referring to authentication information stored in an original information storage part (S4). When the authentication fails (S4, YES), the system control part instructs a plotter to output white paper and the plotter outputs white paper information (S5) and the processing ends (S7). When the authentication does not fail (S4, NO), secret information stored in the original information storage part is outputted (S6) and the processing ends (S7). Since an illegal user is deluded by white paper output as authentication succeed, an atmosphere for decoding the authentication information is not formed.