The light source device 10A has a light-emitting tube (11) (radial light source) and a reflector (12A). A cylindrical heat-conductive member (14A) is attached on an outer surface of a first sealing portion (114A) near a neck portion (121A) of a reflector (12A) in sealing portions (114) of the light-emitting tube (11). The heat-conductive member (14A) is attached along the outer surface of the first sealing portion (114A) with an end thereof being extended to a section near a light-emitting portion (113). A heat-radiation fin (15A) is attached on the other end of the heat-conductive member (14A).