A variable-speed drive device for a power unit to be used in a vehicle equipped with a stopper pin passed through an opening in a master arm for regulating the amount of rotary movement of the master arm. Shock-absorbing device is provided on the master arm or the stopper pin At least a part of the shock-absorbing device is formed to project into the path along which the stopper pin moves relative to the opening of the master arm, from a stopper pin outer surface or from edge portions of a defining edge of the opening in the master arm onto extension of the stopper pin path. As a result, even if the master arm and the stopper pin contact one another at high speed, it is possible to suppress the sound of contact and suppress an increase in the weight of a stopper pin fixing part.