A power supply circuit includes a slave mode in which a power supply is turned on/off by an external device without any standby power supply and a master mode of turning on/off the power supply circuit in the slave mode. A power relay is provided in the power supply circuit between a power supply and a load circuit. The power supply circuit supplies a drive voltage to the power relay upon reception of a power-on command from an external device. A power supply input terminal receives the drive voltage for driving the power relay.