PURPOSE: To prevent an arc shape coil spring from any deformation even when such processing as heat treatment, shot peening, etc., is carried out.

CONSTITUTION: After forming a coil spring in an arc shape, a pitch in outer peripheral side is expanded at every equal angle. Thereby, a pitch expanded place becomes a bent point 20 and the spring 11 is formed in a polygon shape making each bent point 20 to a summit. Consequently, as each bent point 20 constitutes a node point and rigidity for bending is increased, thereafter, even if heat treatment, etc., are carried out, such deformation as undulation and bent are prevented in advance.