PURPOSE: To protect an inverter using a simple configuration at a low cost by sensing half-wave discharge originating from irregular shape of discharge lamp, controlling oscillation of the inverter, and thereby eliminating use of any insulating element or high withstand voltage component(s) for working well with a higher voltage of the discharge lamp.

CONSTITUTION: In normal discharging, the voltage generated in a capacitor 7 installed between a discharge lamp load circuit and a DC power supply 1 assumes sine waves, and no current flows in diodes 8 and 9. However, a discharge lamp goes in abnormal lighting, and if it makes a half-wave discharge, the voltage generated in the capacitor 7 assumes such a waveform due to the rectifying action of a diode (d) that a DC component is superimposed on the DC power supply 1 in case discharge lamp 4 is expressed by an equivalent circuit consisting of resistances r1, r2 and diode (d). With this superimposed DC component, current flows through the diode 8 or 9, and a control circuit 12 stops oscillation of an inverter 2 with a voltage generated in an impedance element 10 or 11, and thus protection of inverter is achieved.