A torque transmission system has driving and driven pulley units drivingly connected by an endless V-belt. Each of the driving and driven pulley units includes a stationary pulley rotatable with a shaft and a movable pulley disposed to define with the stationary pulley a circumferential continuous groove for receiving the V-belt. The movable pulleys of the driving and driven pulley units are axially resiliently biased so that the radii of the circles 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 of the driven pulley unit is axially shifted by fly weights. The driving and driven pulley units are provided with cam mechanisms each comprising a cam member and a follower member. When the load on the driven pulley unit is suddenly varied, the cam mechanisms are operative to keep constant the rotational speed of the driven pulley unit irrespective of variations in the load on the driven pulley unit.