An object is to provide a method and an apparatus for improving a residual stress in a tubular body, which are enabled to improve the residual stress reliably by clearly defining controlling rage for treatment conditions without depending on an installation state and configuration of the tubular body. When an outer-circumferential surface of a welded portion of a cylindrical tubular body (2) is irradiated with a laser beam that circles around an outer circumference of the tubular body (2), a heating width W in a circumferential direction heated by the laser-beam irradiation and a laser-beam moving speed V in the circumferential direction are set so that a stress in the circumferential direction in an inner surface of the tubular body (2) produced by the heating with the laser beam is at least larger than a yielding stress of a material that the tubular body (2) is made of.