A series load circuit is a circuit formed by connecting a light-emitting device unit 851 (the first load circuit) and a light-emitting device unit 852 (the second load circuit) in series. A voltage generating circuit 111 generates voltage to be applied to the series load circuit. A current detecting circuit 112 detects electric current flowing through the light-emitting device unit 851. A controlling circuit 114 controls the voltage generating circuit 111 so that the electric current detected by the current detecting circuit 112 becomes a predetermined current value.