A pulley structure comprising: a cylindrical first rotating body around which a belt is wound; a second rotating body provided on an inner side of the first rotating body to be relatively rotatable with respect to the first rotating body; a torsion coil spring accommodated in a spring accommodation space formed between the first rotating body and the second rotating body; an end cap that blocks an opening portion on one side of the first rotating body in a rotating axis direction; a rolling bearing provided to be interposed between the first rotating body and the second rotating body on the other side in the rotating axis direction; and wherein the first rotating body has a recessed portion formed on an inner circumferential surface of the opening portion to extend in a circumferential direction, characterized in that a ventilation path that is formed at a position shifted in a radial direction from the rotating axis of the first rotating body and allows the spring accommodation space and the outside to communicate with each other, wherein at least a part of the ventilation path is formed in the end cap, wherein the end cap blocks the opening portion of the first rotating body such that the spring accommodation space communicates with the outside toward the one side in the rotating axis direction only via the ventilation path, the end cap comprises an elastic portion formed of an elastic body and a rigid portion integrated with the elastic portion and having a higher rigidity than that of the elastic portion, the end cap has a seal portion (53, 54) being in contact with the recessed portion of the first rotating body in an outer edge portion and at least the seal portion is formed of the elastic portion, the outer edge portion of the end cap is fitted to the recessed portion in a state of being compressed in the radial direction, wherein the ventilation path comprises at least one cutout ventilation path (116, 117) formed by cutting out a part of the seal portion of the end cap, wherein at least a part of the outer edge portion protrudes further to the one side in the rotating axis direction than the recessed portion. a cylindrical first rotating body around which a belt is wound; a second rotating body provided on an inner side of the first rotating body to be relatively rotatable with respect to the first rotating body; a torsion coil spring accommodated in a spring accommodation space formed between the first rotating body and the second rotating body; an end cap that blocks an opening portion on one side of the first rotating body in a rotating axis direction; a rolling bearing provided to be interposed between the first rotating body and the second rotating body on the other side in the rotating axis direction; and wherein the first rotating body has a recessed portion formed on an inner circumferential surface of the opening portion to extend in a circumferential direction, characterized in that a ventilation path that is formed at a position shifted in a radial direction from the rotating axis of the first rotating body and allows the spring accommodation space and the outside to communicate with each other, wherein at least a part of the ventilation path is formed in the end cap, wherein the end cap blocks the opening portion of the first rotating body such that the spring accommodation space communicates with the outside toward the one side in the rotating axis direction only via the ventilation path, the end cap comprises an elastic portion formed of an elastic body and a rigid portion integrated with the elastic portion and having a higher rigidity than that of the elastic portion, the end cap has a seal portion (53, 54) being in contact with the recessed portion of the first rotating body in an outer edge portion and at least the seal portion is formed of the elastic portion, the outer edge portion of the end cap is fitted to the recessed portion in a state of being compressed in the radial direction, wherein the ventilation path comprises at least one cutout ventilation path (116, 117) formed by cutting out a part of the seal portion of the end cap, wherein at least a part of the outer edge portion protrudes further to the one side in the rotating axis direction than the recessed portion.