A light source device includes a light-emitting tube and a reflector. The light-emitting tube has a light-emitting portion that generates a light beam by an electric discharge between electrodes, and sealing portions provided on both sides of the light-emitting portion. The light-emitting tube has a sub-reflection mirror that covers substantially a front half of the light-emitting portion. The reflector has a neck portion provided with an insertion hole to which the light-emitting tube is inserted, and a reflecting portion integrally formed with the neck portion and having an ellipsoidal curved reflecting surface that irradiates the light beam emitted by the light-emitting portion to align the light beam in a predetermined direction. The opening diameter of the insertion hole on the side of the reflecting surface is greater than the external diameter of the sub-reflection mirror and within the internal diameter of a valid reflection area of the reflector.