To provide an optimal light source unit for a liquid crystal projector providing desired lamp characteristics by cooling the interior of the light source unit while preventing excessive cooling at the lower part of a discharge container of a discharge lamp regardless of the service state of the liquid crystal projector.

In this light source unit 10, ventilating holes 11, 12 are provided to allow cooling air to flow into a concave reflecting mirror 20 from the front side of the light source unit 10 and to be exhausted from the vicinity of a neck part 24, and the discharge lamp 20 is arranged in the coinciding state of the optical axis of the concave reflecting mirror 20 and the optical axis of the discharge lamp 30 so as to light the discharge lamp 20 in a nearly horizontal state. Further, an external lead rod 33 is rotatably provided with a cooling air shielding member 60 for shielding the cooling air flowing toward the lower part of a light emitting part 31.