PURPOSE: To increase the lighting efficiency and enable an increase in output in the full-lighting by inverting the electrode of the applied voltage to each filament of an electric-discharge lamp in the full-lighting, and switching and controlling the electric power for heating the filament in the light control lighting.

CONSTITUTION: An electric-discharge lamp 4 has filaments 4A, 4B, a high frequency of the same frequency is supplied to the both ends of the electric- discharge lamp 4 and each filament 4A, 4B, and the electric power of high frequency supplied to the both ends of the electric-discharge lamp 4 is regulated by a light control switch S. During the half cycle of the lamp current in the complete lighting, the electrode of the applied voltage to each filament 4A, 4B is inverted at the time when the integral value of the lamp current reaches nearly the half, and further the filament heating power is switched and controlled by switches S1, S2 during the operation of the switch S to increase the lighting efficiency.