the fatigue strength did not reach about 600 mpa as shown in fig9 in a case that a shot particle size is 0 1 mm the fatigue strength being the same level as that about 600 mpa of a carburized and hardened steel material could be obtained u201c u25ef u201d of fig9 however in an another case that a particle size is 0 07 mm or 0 01 mm the bending fatigue strength was 600 mpa or smaller u201c u00d7 u201d of fig9 from results of experimental example 5 fig9 it was found that in the second shot peening treatment a shot particle size should be set to 0 1 mm or larger it is assumed that when a particle size of shots used in the second shot peening treatment is small only a surface of a cast iron is smoothened the compressive residual stress of the outermost surface of a steel material was not generated and the fatigue strength could not be improved gears gears on which the first to third shot peening treatments were performed z being manufactured with a test material of experimental example 1 and gears y being manufactured with a test material to which the third shot peening treatment was not applied were prepared.