In a light source device provided with a light emission tube in which a light emitting element is enclosed and at least one laser oscillator part for radiating a laser beam towards said light emission tube, for focusing a beam within a light emission tube with a large solid angle and for preventing that the beam with a high energy density impinges upon the wall of the light emission tube, the light emission tube has a tube wall, part of which is made to function as a focusing means, or the light emission tube has a focusing means at the inner surface thereof.