An interface circuit including an interface controller operated by at least a system power supply or an external power supply. A switch is connected between an interface controller and a system power circuit for conducting system power. The interface controller includes a control signal generating circuit for generating a control signal that closes the switch at a timing that does not affect data transmission performed by the interface circuit in accordance with a trigger signal, which indicates that the system power has gone on. Similarly, the switch is opened at a time when data transmission will not be affected.