PURPOSE: To improve the fatigue strength of a steel member and to prevent the breakage of corners by subjecting a steel member having corners, such as gear, after carburizing, quenching, and tempering to two-stage shot peening of different high hardness shots.

CONSTITUTION: The surface of parts made of steel having corners, such as gear, used for machinery is carburized and subjected to quench-and-temper treatment, by which the strength of the corners is improved. Further, in order to improve the fatigue strength of the corners, shots having >700 hardness HV and also having relatively large grain size of 0.6-1.2mm are sprayed onto the surface of the gear to perform first shot peening. Then, annealing is done at 100-200°C, and second shot peening is performed by using shot grains of ≥700 hardness HV and 0.1-0.5mm grain size. By this method, proper compressive residual stress distribution can be formed in the gear, and the gear improved in fatigue strength and free from breakage of corners can be obtained.