A light source device includes: a light source lamp that has a light-emitting portion and sealing sections provided on both sides of the light-emitting portion; a reflector that aligns and irradiates a light beam radiated by the light source lamp in a predetermined direction; and a sub-reflection mirror of which reflection surface opposes to a reflection surface of the reflector and reflects the light beam radiated by the light-emitting portion of the light source lamp toward the reflector. In the sub-reflection mirror, an adhesion surface of an insertion hole is bonded to the sealing section by an adhesive in a condition that the sealing section is inserted in the insertion hole. A cutting portion of the sub-reflection mirror is so formed to extend over a portion bonded by the adhesive in plan view.