A light source device is provided. The light source device has: a discharge lamp that emits light by discharge in an interelectrode space between a first electrode and a second electrode; a main mirror arranged at the first electrode side that reflects light flux generated by the discharge in the interelectrode space so as to emit the light flux to an area to be illuminated; a sub-mirror arranged at the second electrode side facing opposite the main mirror that reflects the light flux from the interelectrode space toward the interelectrode space; and a driver that supplies alternating current to the first and the second electrodes, and changes duty ratio of the alternating current supplied to the first and the second electrodes in accordance with a predetermined pattern. In the light source device, maximum value of time ratio of a first anode period in which the first electrode works as anode to one cycle of the alternating current is larger than maximum value of time ratio of a second anode period in which the second electrode works as anode to the one cycle of the alternating current.