Manufacturing method for coil spring

A manufacturing method for a coil spring, comprising a first shot peening process and a second shot peening process to be performed after the first shot peening process,

• the first shot peening process comprising causing a first shot to impinge on a spring wire at a first projectile speed, thereby producing a compressive residual stress such that a peak part of the compressive residual stress exists within the spring wire,
• the second shot peening process comprising causing a second shot to impinge on the spring wire at a second projectile speed lower than the first projectile speed and with kinetic energy lower than that of the first shot, thereby increasing the compressive residual stress in a region near the surface to be higher than the peak part of the compressive residual stress.