A pulley structure comprising: a cylindrical outer rotating body around which a belt can be wound; an inner rotating body provided on the inside of the outer rotating body and relatively rotatable with respect to the outer rotating body around a common rotation axis with the outer rotating body; and a coil spring provided between the outer rotating body and the inner rotating body, wherein the pulley structure further comprises a lock mechanism configured such that further torsional deformation in a diameter expansion direction of the coil spring is restricted and the outer rotating body and the inner rotating body rotate integrally with the coil spring when an outer circumferential surface of a free part of the coil spring abuts against the outer rotary body due to diameter expansion of the coil spring, wherein the coil spring functions as a one-way clutch which is engaged with each of the outer rotating body and the inner rotating body due to torsional deformation in the diameter expansion direction to transmit torque between the outer rotating body and the inner rotating body when the inner rotating body relatively rotates with respect to the outer rotating body in a forward direction, and which slides with respect to at least one of the outer rotating body and the inner rotating body due to torsional deformation in a diameter reduction direction and does not transmit torque between the outer rotating body and the inner rotating body when the inner rotating body relatively rotates with respect to the outer rotating body in a reverse direction, wherein a spring wire of the coil spring has a trapezoidal shape in a cross-section along a direction passing through the rotation axis and parallel to the rotation axis, and has a rotation axis direction length Ti [mm] at an inner diameter side part on the cross-section longer than a rotation axis direction length To [mm] at an outer diameter side part on the cross-section, characterized in that, when the number of windings of the coil spring is N, the following expression is satisfied: N × Ti − To / 2 < 1, wherein the coil spring has a gap between adjacent spring wires in the rotation axis direction, and wherein the coil spring has a wire inclination of 1° or less. a cylindrical outer rotating body around which a belt can be wound; an inner rotating body provided on the inside of the outer rotating body and relatively rotatable with respect to the outer rotating body around a common rotation axis with the outer rotating body; and a coil spring provided between the outer rotating body and the inner rotating body, wherein the pulley structure further comprises a lock mechanism configured such that further torsional deformation in a diameter expansion direction of the coil spring is restricted and the outer rotating body and the inner rotating body rotate integrally with the coil spring when an outer circumferential surface of a free part of the coil spring abuts against the outer rotary body due to diameter expansion of the coil spring, wherein the coil spring functions as a one-way clutch which is engaged with each of the outer rotating body and the inner rotating body due to torsional deformation in the diameter expansion direction to transmit torque between the outer rotating body and the inner rotating body when the inner rotating body relatively rotates with respect to the outer rotating body in a forward direction, and which slides with respect to at least one of the outer rotating body and the inner rotating body due to torsional deformation in a diameter reduction direction and does not transmit torque between the outer rotating body and the inner rotating body when the inner rotating body relatively rotates with respect to the outer rotating body in a reverse direction, wherein a spring wire of the coil spring has a trapezoidal shape in a cross-section along a direction passing through the rotation axis and parallel to the rotation axis, and has a rotation axis direction length Ti [mm] at an inner diameter side part on the cross-section longer than a rotation axis direction length To [mm] at an outer diameter side part on the cross-section, characterized in that, when the number of windings of the coil spring is N, the following expression is satisfied: N × Ti − To / 2 < 1, wherein the coil spring has a gap between adjacent spring wires in the rotation axis direction, and wherein the coil spring has a wire inclination of 1° or less.