A torque transmission system has driving and driven pulley units around which an endless V-belt extends. Each pulley unit has a stationary pulley part rotatable with a shaft and a movable pulley part disposed to define with the stationary pulley part a circumferentially continuous groove for receiving the V-belt. The movable pulley parts of the pulley units are axially resiliently biased so that the radii of the circle along which the V-belt engages the driving and driven pulley units are varied to change the speed-change ratio of the transmission system. The movable pulley part of the driven pulley unit is axially shifted by the action of fly weights held between a stationary plate fixed to the shaft and a holder fixed to the movable pulley part. One of the holder, movable pulley part and stationary plate is provided with surfaces extending parallel to the axis of the shaft and directed radially inwardly to radially inwardly support the fly weights when they are centrifugally moved to their radially outermost positions whereby the axial thrust force imparted by the radially outward movement of the fly weights to the V-belt is limited.