A switch device controls a voltage applied to a load element according to an electric power supply from a power source. A driver circuit controls activation and deactivation of the switch device. A circuit portion sets a duty ratio and output the duty ratio to the driver circuit for causing the driver circuit to activate the switch device according to the duty ratio to control a voltage applied to the load element. The circuit portion sets the duty ratio according to a duty ratio characteristic curve. In the duty ratio characteristic curve, the duty ratio decreases gradually with increase in a voltage of the power source, and decrease in the duty ratio with increase in the voltage of the power source becomes small gradually.