A switching element is switched at a high frequency in accordance with control by a pulse generating circuit by using a D.C. power source portion as a power source, which results in that a discharge lamp is driven with a D.C. current through a smoothing circuit. A current detecting resistor detects the lamp current caused to flow through the discharge lamp, a current controlling circuit amplifies a deviation between the detected lamp current value and a preset current reference value, and the resulting value is applied to a feedback terminal (F/B) of the pulse generating circuit through a gain setting circuit. A gain changing circuit changes a loop gain of a feedback control system including the switching element, the smoothing circuit, the discharge lamp, and the pulse generating circuit at an output point of the gain setting circuit in correspondence to a fluctuation in a power source voltage (Vp) from the D.C. power source portion.