 with the driving method for supplying the low-frequency alternating current. On the other hand, with the driving method for supplying the low-frequency alternating current, blackening, devitrification, or the like of the discharge lamp main body occurs and the life of the discharge lamp decreases. In the driving method for supplying the high-frequency alternating current, when the discharge lamp is lit, the electrodes are heated by an arc discharge that occurs between the electrodes. The electrodes melt and the distance between the electrodes gradually increases. On the other hand, with the driving method for supplying the low-frequency square wave alternating current, blackening, devitrification, or the like of the discharge lamp main body occurs and the life of the discharge lamp decreases. 