An electronic ballast for a discharge lamps has a dimming control for varying an input DC voltage and an input driving frequency in combination to achieve a lamp dimming in accordance with a dimmer demand. The ballast includes a dimming processor with a memory for storing a table that gives a predetermined relation between the dimming demand and a voltage command designating the input DC voltage as well as a frequency command designating the input driving frequency. A command processing unit is included in the processor to derive the voltage and frequency commands that matches with the dimmer demand so as to vary the light output of the lamp in accordance with the voltage command and the frequency command, respectively derived from the table.