An image forming apparatus 1 (a) displays a visible code in which network identification information of the image forming apparatus 1 is encoded and a function code corresponding to a function of the image forming apparatus 1, (b) transmits function designation screen data to the portable terminal device 2 connected by using the network identification information, (c) receives a function code input to a function designation screen based on the function designation screen data from the portable terminal device 2 via a network, (d) determines whether the received function code matches the displayed function code or not, (e1) transmits operation screen data corresponding to the function code to the portable terminal device 2 via the network when both match, and (e2) does not transmit operation screen data to the portable terminal device 2 when both do not match.