A discharge lamp ballast apparatus has a reflecting mirror 2 and a power source circuit 5. The reflecting mirror 2 is disposed around a discharge light bulb 1 in such a manner as to cast light from the discharge light bulb 1 in one direction. The power source circuit 5 applies a start pulse of a negative potential with respect to the potential of the reflecting mirror 2 to an electrode 6 located at a side with the higher electric field concentration produced between electrodes 6 and 7 to which a high voltage of the start pulse is applied. This makes it possible to produce a dielectric breakdown near the electrode 6, and makes it easier to start the discharge light bulb 1.