An electric ballast attached to the outside of a lighting system. The system comprises a lamp housing with front and rear openings, a front lens closing the front opening, a discharge lamp put in the housing, and a reflector that reflects light of the lamp toward the lens. The ballast comprises: a ballast housing which closes the rear opening and is connected with frame ground via at least one of the lamp housing and the reflector; a lamp socket and an igniter, put in the front of the ballast housing; and a power converter put in the ballast housing. The output of the converter is connected with the socket, which directly connects with the lamp, and the igniter is connected between them. Heat generating parts of the converter are put in the rear of the ballast housing, and the rear functions as a radiator.