PURPOSE: To improve the fatigue strength of a spring by performing shot peening treatment while projecting a refrigerant onto a coil spring and applying specific average compressive residual stress to the surface layer.

CONSTITUTION: A coil spring is formed by using a carbon steel wire or alloy steel wire having ≥550 base-material hardness HV. Shot peening is applied to this coil spring while projecting a refrigerant of about -20 to -100°C. By this treatment, an average compressive residual stress of ≥80kgf/mm2 can be applied at least to the depth of 150μm from the outermost layer. Because ruggedness is brought about to some extent by means of shot peening treatment, the fatigue strength improving effect can further be increased by performing surface smoothing, such as barrel polishing.