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 a cylindrical tubular body (2) is improved in its residual stress by locally irradiating an outer-circumferential surface of a welded portion (C) of the tubular body (2) with laser beams (5a) and by moving an irradiation area (s) in an circumferential direction, a plurality thermocouples (9) are installed on the tubular body (2) to be improved, a temperature history of the outer surface of the tubular body (2) by the irradiation of the laser beam (5a) is managed by measuring the temperature history itself.