PURPOSE: To reinforce the inner peripheral surface of a coil spring and to increase the fatigue strength by shot-peening the inner peripheral surface at least of the coil spring.

CONSTITUTION: The inner peripheral surface at least of a coil spring (valve spring) 1 subjected to forming is subjected to shot-peening. The inner peripheral surface at least of the coil spring 1 subjected to shot-peening is subjected to a magnetic polishing by a magnetic abrasive 6. The fatigue strength of the peripheral wall face 13 of the spring 1 can be improved.