PURPOSE: To remove the unstability of the arc caused by an acoustic resonance phenomenon by making the ratio of DC operation period to high frequency operation period according to the light control ratio.

CONSTITUTION: A power source A switches DC power sources V1, V2 to supply a DC current to a high-voltage electric-discharge lamp 1, and a power source B switches the power source V1, V2 to supply a high frequency power to the electric-discharge lamp 1. The power source B is intermittently operated by a control circuit 3, and the power source A is operated during the non-operation period of the power source B. In this case, a light control signal from an input means 2 is inputted to the control circuit 3, and the switching element S1 or S2 of the power source A or B is controlled by a PWM control means 4 to control the supply power to the electric-discharge lamp 1, and the ratio of operation period of the power source A to that of the power source B is made variable according to the light control ratio of the discharge lamp 1 by an operation period control means consisting of a light control level judging circuit 6 and a low frequency oscillating circuit 5.