An electronic ballast for a discharge lamp is capable of dimming the lamp over a wide range, yet assuring to operate the lamp efficiently with a minimum lamp power variation in compensation for a possible lamp characteristic variation. The ballast provides an unique voltage-wattage output characteristic which varies with a varying dimming ratio for reducing the lamp power as the dimming ratio decreases. The output characteristic is designed to follow a true lamp characteristic that the lamp voltage will decrease as the lamp power is reduced to a certain level. With this result, it is possible to give a wide voltage range within which the lamp power being supplied to the lamp is kept substantially constant irrespective of the lamp characteristic variation, thereby assuring to reduce the lamp power exactly as intended by the dimming ratio even in the presence of the lamp characteristic variation.