A method for manufacturing a discharge tube, including heating a quartz tube by irradiation and closing it shut, is presented, in which variation of the heating temperature of the quartz tube can be reduced. Laser light is irradiated on a portion of the quartz tube, and in the step of sealing the portion of the quartz tube onto which laser light has been irradiated, the laser light is scanned while oscillating back and forth. The intensity of the laser light is controlled such that the intensity of the laser light when the size of a displacement of the laser light is maximal is smaller than the intensity of the laser light when the size of the displacement of the laser light is minimal.