To provide an interface device and method of controlling power to achieve reduction in power consumption by presetting the transceiver in the standby state. When the physical layer (PHY) 10 makes a transition to a low power consumption mode, a standby signal (standby) for instructing the optical transceiver 30 to preset it into the low power consumption mode is brought into active and is output thereto. When the optical transceiver receives said standby signal, it is brought into the standby mode which is the low power consumption mode.