A light source device, includes: a light source lamp that has a light emitting tube and a pair of electrodes, the light emitting tube including a discharge space; the pair of electrodes being disposed in the discharge space of the light emitting tube; a reflector in a substantially C-shape in cross section that reflects a light beam irradiated from the light source lamp; and an unused-light reflecting member being disposed on a light irradiation front side of the reflector and including an unused-light reflecting surface, the unused-light reflecting surface reflecting unused light irradiated outside the reflector of the light beam irradiated from the light source lamp, the unused-light reflecting surface being an ellipsoidal surface that has a first focal position on a light emission center of the light source lamp and a second focal position onto which the ellipsoidal surface reflects and converges the unused light to be guided to the outside of the light source device.