To adjust luminance up to a low-luminance part with one external dimming knob by providing a voltage adjusting process circuit which turns on a 2nd switching (TR) below an actuation voltage and supplies a current to a resistor for voltage division to lower the voltage applied to an LED.

The voltage V0 is varied by adjusting a variable resistor 3 for dimming with the external dimming knob to vary the duty cycle of a pulse output by MPU in a pulse generating circuit 1. Then when the voltage V0 drops to an arbitrary voltage V2, software varies the duty cycle of the pulse output of the pulse generating circuit 1 to its limit and the voltage adjusting circuit 2 is actuated at this time to turn on the switching TR Q2 with its output, thereby lowering the voltage V1 applied to the LED by a current limiting resistor R1 and the resistor R2 for voltage division. Here, the duty cycle of the pulse generating circuit 1 is adjusted again.