PURPOSE: To lower the effective value of no load secondary voltage.

CONSTITUTION: An inverter circuit 1, which includes switching elements Q1 and Q2 being switched in a high frequency manner, converts DC power E into AC of rectangular waves. This unit is equipped with a filter circuit comprising a capacitor C1, which prevents a high frequency current from flowing to the discharge lamp DL connected to the output of an inverter circuit 1, and an inductor element L2. A switching element Q5 is provided inside the closed loop composed of a capacitor C1, the discharge lamp DL, and an inductance element L2. When a lamp voltage detecting circuit 3 detects no-load condition, a control circuit 2 turns off all the switching elements Q1-Q4 of an inverter circuit 1, in the specified period of the half cycle of the said rectangular wave AC, and also turns off the switching element Q5. Hereby, the said closed loop is broken, and the period of the voltage across the capacitor C1 being applied to the discharge lamp DL is shortened, and the effective value of the no-load secondary voltage is lowered.