PURPOSE: To properly start and light a low-pressure mercury lamp, regardless of the length of a cable for connecting the lamp, by applying the constitution that the lamp starts and lights at DC voltage doubled through a voltage doubler rectifier circuit after receipt of the voltage from an active filter circuit.

CONSTITUTION: When the commercial power supply 1 is turned on, an active filter circuit 3 operates, and the preset DV voltage is generated across a capacitor. At the same time, the transistor(Tr) 4 of a step-down chopper circuit 4 also begins a switching operation, thereby charging a capacitor 44. Also, a set of Tr 51 and Tr 54, and a set of Tr 52 and Tr 53 in a rectangular wave voltage generation circuit 5 are repeatedly turned of and off alternately, and consequently the voltage of the capacitor 44 of the circuit 4, and the voltage of the capacitors 5E and 5F of the circuit 5 become nearly equal to the voltage of the capacitor 34 of the circuit 3, thereby lighting a low-pressure mercury lamp 6 with direct current. When the contact points of relays 5G and 5H are selected to the normally open side after the elapse of a certain time, the lamp 6 lights with rectangular wave AC and keeps stable charge.