<p>To prevent an increase in the diameter of a shift drum and eliminate the significant change of the shape of a lead groove even if the number of shift positions set on the shift drum is increased. A shift drum is provided on the outer circumference thereof with at least one particular lead groove among lead grooves which continuously extends beyond one circle of the shift drum and with both ends of the second lead groove which are arranged so as to be out of alignment with each other in the axial direction of the shift drum. A common shift position adapted to establish gear trains with two shift stages different from each other is set at one position corresponding to both the ends of the particular lead groove along the circumferential direction of the shift drum.</p>
