PURPOSE: To reduce the loss, and while to stably dim the light in a wide range by controlling the duty value of the heating current so as to be increased in response to the reduction of the duty value of the discharging voltage.

CONSTITUTION: A duty variable oscillator 62 generates the direct pulse current at a duty value corresponding to the output voltage of an OP amplifier 61. A MOS-FET 63 performs the switching at a high speed between drain.lease in response to the output of the oscillator 62. The direct pulse current corresponding to the output of the oscillator 62 is flown to a heating circuit, and the voltage Vin is charged to the duty value of the heating current in the direction opposite to the heating current. The duty value of the heating current is controlled so as to be increased in response to the reduction of the duty value of the discharging current. A common mode choke coil 30 does not have the self-inductance to a route of the heating current, and has a high impedance between filaments 11, 12. Bypass of the discharging current through the coil 30 is not generated.