To eliminate a lighting miss when a discharge lamp is lighted by starting AC lighting after DC lighting has been kept on for the definite period of time since it was started, and furthermore applying electric power which is the sum of rated electric power and definite electric power, for the period of time which is the sum of the aforesaid hold time and a definite period of time.

A polarity change-over switching circuit 7 keeps on the conditions of turning switching elements 71 and 74 on and switching elements 72 and 73 off for the period of time shorter than TB=T1, which represents time having elapsed since a discharge lamp 6 was lighted by a signal from a lighting timer 9, and the DC lighting of the lamp 6 is kept on in the direction indicated by an arrow head B. The aforesaid time T1 is set to be the shortest time capable of preventing the occurrence of any lighting miss. Next, an electric power control means 8 keeps on prescribed electric power which is initially indicated electric power for the period of time TB=T2 longer than time T1. Next, after time T2 has elapsed, control is executed similarly to control in the part, however, control is executed smoothly in such a way that electric power is not abruptly lowered from the initial indicated electric power. And time T2 is also set to be as short as possible. By this constitution, the lamp 6 can thereby be prevented from being missed in lighting directly after starting where a lighting condition is unstable.