A printer (image forming device) (10), which includes AC loads driven by an AC power, is connected to an external device (30) via a single power line for a logic operation and a signal line. More specifically, a printer driver (11) and an engine controller (13) operate on a 5V power supply voltage obtained via a USB cable (20). When a standby state is entered, a relay (17) is turned off, an AC power supply to a drive power supply (19) is disconnected, and a power supply to the AC loads is disconnected. When the standby state is released by a signal received from the PC (30), an ON signal is generated for the relay (17) to supply the AC power. In this way, the standby power is reduced to improve energy saving.