By controlling the thickness of the nitrization layer formed on the surface, the coil spring made of spring steel may be made highly resistant to permanent set and fatigue. By carbonitriding the material of the coil spring to a depth of 20 to 50 .mu.m from the surface thereof so as to have an internal hardness of Hv 540 or higher, the coil spring can be made to endure 10.sup.7 cycles of repeated high stress without suffering any significant permanent set.