PURPOSE: A method is provided to improve the fatigue strength by manufacturing a plurality of samples for the fatigue testing of a Cr-Mo steel degraded material which is used as a steel for a high temperature and high pressure container for a long time as changing an irradiation density with parameters such as laser output, irradiation speed of a laser beam irradiator and diameter of an irradiation beam. CONSTITUTION: The method for increasing the fatigue strength of a steel for a pressure container comprises the processes of irradiating laser on a matrix steel for a pressure container which is easily subject to degradation, thereby inducing the phase transformation of a surface hardening layer of martensite with a heat source of the irradiated laser; and generating residual compression stress inside the steel with the phase transformation so that impurities segregated into the grain boundary of the steel are diffused by a high temperature heat source of the laser irradiation resulting in an increase of the fatigue strength of the steel.