Hollow spring and method of manufacturing the same

A method of manufacturing a hollow spring, comprising:

• providing a steel tube processed by bending to a predetermined shape used for the hollow spring;
• and
• applying compressive force to at least a portion of an outer surface of the steel tube, including a portion processed by bending, from a circumferential direction to apply compressive residual stress to at least a portion of an inner surface of the steel tube;
• wherein
• applying the compressive force to the outer surface of the steel tube comprises pressing the steel tube with a die,
• the die comprises a pressing surface shaped such that the compressive force can be applied to the at least the portion of the outer surface of the steel tube, including the portion processed by bending, from the circumferential direction,
• the pressing surface extends in the circumferential direction along the outer surface of the steel tube,
• the pressing surface extends in the circumferential direction of the steel tube over a half of a circumference, and
• a fatigue life of the steel tube is prolonged by applying the compressive residual stress to the inner surface of the steel tube.