A master device supplies a power supply voltage through a power transmission line to a slave device that generates an operation signal indicating the contents of an operation received by an operation unit. The slave device includes an operation execution switch that is turned on to apply a reference voltage to the power transmission line, upon reception of the operation by the operation unit. The master device determines whether or not a current accompanying a pilot signal is flowing through the power transmission line, while intermittently sending the pilot signal to the power transmission line in the state of stopping the supplying of the power supply voltage to the slave device. If it is determined that the current is flowing, the master device supplies the power supply voltage to the slave device.