Abstract: Rotation speed of automotive auxiliary machines is controlled by a drive pulley provided on a crank shaft and a driven pulley communicating with the drive pulley through a V-belt. The drive pulley comprises a rigid pulley secured on a drive shaft, a guide holder to be rotated together with the drive shaft by a mount bolt, and a slide pulley axially-movably located between the guide holder and the rigid pulley. The slide pulley provides a power piston to be moved along a guide holder shaft. The power piston, the guide holder flange, and the slide pulley define a power piston chamber.Inside of the power piston chamber are located a flyweight to be moved by centrifugal force and a servo valve to be axially moved by the flyweight. The servo valve is fitted on the outer peripheral surface of the power piston.