Abstract: A coil spring has a spring wire that is wound in a coil shape and whose sectional outer basic shape is circular and flat faces are formed between coil inner and outer diameter sections of a sectional outer shape of the spring wire, wherein a flattening T/W of the sectional outer shape with the flat faces in the coil axis direction is set so that a stress ratio is equal to or lower than a stress ratio of a coil spring whose sectional outer shape is circular.

Abstract: A stress distribution state is improved and a better design is realized. A coil spring 1 has a spring wire 3 that is wound in a coil shape and whose sectional outer basic shape is circular. Between coil inner and outer diameter sections 5 and 7 of a sectional outer shape of the spring wire 3, flat faces 9 and 11 are formed so that coil portions adjacent to each other in the direction of a coil axis 4 are stabilized when they are brought into contact with each other. A flattening T/W of the sectional outer shape with the flat faces 9 and 11 in the coil axis direction is set so that a stress ratio is equal to or lower than a stress ratio of a coil spring whose sectional outer shape is circular. The flat faces 9 and 11 stably receive load and shorten a solid length in a tightly compressed state. This is advantageous in designing a spring having a longer stroke and lower rigidity.