A light source device (100) includes a discharge lamp (1), a reflection mirror (2), a transparent plate (3) and a case (7). The reflection mirror (2) has inlet openings (4a, 4b) and outlet openings (5a, 5b) formed in the vicinity of the front end of the reflection mirror (2). The case (7) has outlet openings (7a, 7b) disposed on the front side of the reflection mirror (2), and an inlet opening (7c) disposed on the rear side of the reflection mirror (2). The inlet opening (7c) of the case (7) allows an air to flow into the case (7) and to flow along a back surface (2b) of the reflection mirror (2). The inlet openings (4a, 4b) of the reflection mirror (2) allow the air to flow into the interior of the reflection mirror (2). The outlet openings (5a, 5b) of the reflection mirror (2) and the outlet openings (7a, 7b) of the case (7) allow the air to be exhausted out of the case (7).