Crankshaft and method of strengthening shaft component

A strengthening method for a shaft member having a cross-sectional circular shape portion, wherein compressive residual stress is applied to a portion on the cross-sectional circular shape portion in which bending load and/or torsional stress to be applied in a circumferential direction is greatest during use of the shaft member, by pressing said portion from a normal line direction using a pressing member having a pressing surface comprising an approximately cylindrical curved surface, and which has radius of curvature greater than that of the cross-sectional circular shape portion,

• wherein relief portions are formed at both sides in a circumferential direction of the pressing surface, the relief portions have an approximately circular arc shape when viewed in an axial direction of the cross-sectional circular shape portion while the pressing surface presses the portion, the relief portions project toward an opposite direction with respect to a concave direction of the pressing surface, the relief portions are gradually separated in a radial direction from the portion toward the circumferential direction while the pressing surface presses the portion, and the compressive residual stress in the relief portions is decreased in a state in which the pressing surface presses the portion.