Provided is a steel wire for a spring in which sag resistance and fatigue characteristics can be improved through the manufacturing process without relying on the addition of an alloy element. A spring having a structure obtained by quenching and tempering, the spring comprising a first layer on the surface, a second layer disposed further inward from the first layer, and a third layer disposed further inward from the second layer so as to extend to the center, the second layer having a lower hardness than the first and third layers.