The polarity of a voltage applied to the electrode pair is inverted by a changing signal generated at every boundary area of each filter color of the color wheel, and also in the blue color region (B) (a, b. c). Herewith, time during which the lamp electrode is positive and that negative is equal to each other every time the color wheel makes two rotations (T), thereby maintaining the heat load to be evenly distributed between both of the electrodes.