Abstract: An image forming apparatus includes: an image reading device for reading an original document image to form the image in a copying mode; a modem device for decoding data sent from a public network; a first memory for registering telephone identification information for a remote station; a detector for detecting a telephone identification provided with a caller identification service of a communication enterprise; and a controller for controlling the modem device, realizing a first communication mode for receiving facsimile data to form the image in a facsimile mode, or a second communication mode for diagnosis of the apparatus and for setting control information of the apparatus from the remote station through the modem device. The controller realizes the first communication mode when the telephone identification does not coincide with the registered telephone identification, and realizes the second communication mode when the telephone identification coincides with the registered telephone identification.