A reference voltage source generates a reference voltage for adjusting an electric current for a dimming operation. The first current source generates a first current. A first current mirror circuit includes multiple output terminals, duplicates the first current, and outputs multiple first duplicated currents via the multiple output terminals. Multiple first switches are provided on paths for the respective multiple first duplicated currents. A converting resistor, with one terminal set to a fixed electric potential, is provided on a path for the multiple first duplicated currents output from the first current mirror circuit. A decoder circuit receives a control signal from an external circuit, and controls the ON / OFF operations of the multiple first switches. The reference voltage source outputs, as the reference voltage, a voltage that corresponds to a voltage drop that occurs at a converting resistor.