PURPOSE: To reduce the speed rise of a driven equipment even if the rotating speed of a driving shaft is increased by providing a variable-diameter pulley which is expanded in diameter by the centrifugal force on the driven side.

CONSTITUTION: When an engine is driven, a driven equipment is driven by a belt 13 and a driven pulley 7. At the low speed of the engine, side faces 27, 27 are curved to the outside, a belt pulley 24 is set to a shrunk state having a small outer diameter D, and the driven equipment is driven at the relatively high sped. When the engine speed becomes high, the diameter is expanded to D by the centrifugal force of the belt pulley 24 itself and the centrifugal force of a mass body 29. Thereby, the speed change ratio is decreased, and the speed rise ratio is reduced. Accordingly, the speed rise of the driven equipment can be reduced even if the rotating speed of the driving shaft is increased.