A variable-speed drive device for a power unit, used for a transmission wherein speed changes are performed by intermittent rotation of a shift drum, comprising: a shift spindle for operating the transmission for speed changes; a master arm fixedly mounted on the shift spindle for rotary movement with the shift spindle and having an opening; and a stopper pin extending through the opening of the master arm to regulate the amount of the rotary movement of the master arm; characterized in that: the master arm is provided thereon with shock absorbing devices; at least a part of each of the shock absorbing devices protrudes into a path (Lo) of rotary movement of the stopper pin in the opening of the master arm, from each of edge portions (72f and 72g) included in a defining edge of the opening of the master arm, when viewed in the axial direction of the stopper pin, said edge portions (72f and 72g) lying on extensions of the path (Lo) of the rotary movement of the stopper pin; the master arm is so formed in a plate shape that abutment of the edge portions (72f and 72g) of the opening of the master arm with the outer surface of the stopper pin regulates the rotary movement; and each of the shock absorbing devices is provided on one of the surfaces of the master arm with respect to the axial direction of the stopper pin; and each of the shock absorbing devices comprises a damper body of an elastic material, and a fixing pin fixedly attaching the damper body to the master arm. a shift spindle for operating the transmission for speed changes; a master arm fixedly mounted on the shift spindle for rotary movement with the shift spindle and having an opening; and a stopper pin extending through the opening of the master arm to regulate the amount of the rotary movement of the master arm; characterized in that: the master arm is provided thereon with shock absorbing devices; at least a part of each of the shock absorbing devices protrudes into a path (Lo) of rotary movement of the stopper pin in the opening of the master arm, from each of edge portions (72f and 72g) included in a defining edge of the opening of the master arm, when viewed in the axial direction of the stopper pin, said edge portions (72f and 72g) lying on extensions of the path (Lo) of the rotary movement of the stopper pin; the master arm is so formed in a plate shape that abutment of the edge portions (72f and 72g) of the opening of the master arm with the outer surface of the stopper pin regulates the rotary movement; and each of the shock absorbing devices is provided on one of the surfaces of the master arm with respect to the axial direction of the stopper pin; and each of the shock absorbing devices comprises a damper body of an elastic material, and a fixing pin fixedly attaching the damper body to the master arm. the master arm is provided thereon with shock absorbing devices; at least a part of each of the shock absorbing devices protrudes into a path (Lo) of rotary movement of the stopper pin in the opening of the master arm, from each of edge portions (72f and 72g) included in a defining edge of the opening of the master arm, when viewed in the axial direction of the stopper pin, said edge portions (72f and 72g) lying on extensions of the path (Lo) of the rotary movement of the stopper pin; the master arm is so formed in a plate shape that abutment of the edge portions (72f and 72g) of the opening of the master arm with the outer surface of the stopper pin regulates the rotary movement; and each of the shock absorbing devices is provided on one of the surfaces of the master arm with respect to the axial direction of the stopper pin; and each of the shock absorbing devices comprises a damper body of an elastic material, and a fixing pin fixedly attaching the damper body to the master arm.