An electronic ballast for a discharge lamp has a controller that increase luminous flux of the lamp to a stabilized level in a constant time irrespective of possible variations in lamp characteristic. The controller utilizes a reference lamp power table and a reference lamp voltage table respectively defining a reference lamp power and a reference voltage varying with time. The controller includes a voltage deviation detector that gives a voltage correction index indicating a voltage deviation between the reference voltage and a monitored lamp voltage. The voltage correction index is processed at an offset provider that gives an offset power. A target power generator is included to correct the reference lamp power in view of the offset power, thereby generating a target lamp power which is constantly updated and is supplied to the lamp.