To restrain degradation of beam-condensing characteristics light flux accompanying a change with time of an electrode.

The light source device includes a discharge tube 10 having a pair of electrodes (an anode 13 and a cathode 14), and an elliptical reflecting mirror 21 reflecting irradiation light from the discharge tube 10. A focal position the elliptical reflecting mirror 21 has is arranged offset in a direction parallel with an electrode axis from a center position between the pair of electrodes. By offsetting the discharge tube 10 in a direction parallel with the electrode axis toward a larger abrasion volume of the electrodes, an attenuation of light utilization efficiency accompanying a change of an arc length caused by the abrasion of the electrodes is lessened, and therefore, it can be made brighter then a conventional light source after a long time of use. Moreover, by setting the offset value of the discharge tube 10 at an optimum value, it is possible to correct degradation of brightness of the other discharge tubes due to melanization or devitrification, so that brightness can be kept constant for long period of time.

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