Abstract: A variable diameter pulley comprises a pair of pulley main bodies (18, 19) connected to a rotating shaft (15), and a power transmission ring (20) interposed between the pulley main bodies. A belt (2) is passed around an outer periphery of the power transmission ring (20). The power transmission ring (20) is made eccentric relative to an axis (15a) of the rotating shaft (15) so as to be varied in its contact diameter (D). Both pulley main bodies (18, 19) are threadedly connected to the rotating shaft (15) by a torque cam mechanism (T) composed of a pair of thread connection mechanisms, which are inversely threaded to each other. Torque for effecting rotation of the respective pulley main bodies (18, 19) relative to the rotating shaft (15) is converted into a force, which causes both pulley main bodies (18, 19) to approach each other in equal amounts of stroke.