of the outer circumference of the shift drum 103 accordingly an unnecessary portion of the shift drum 103 is cut away for the purpose of weight reduction the shift drum 103 is supported to a drum shaft 104 a drum driven gear 105 and a shift cam 106 are also mounted on the drum shaft 104 the shift drum 103 the drum driven gear 105 and the shift cam 106 are joined together by an interlocking pin 107 to restrain their relative rotation e g so that these members 103 105 and 106 are rotated together a shift spindle 108 is rotatably supported to the rear crankcase 26 and the rear crankcase cover 27 a sector gear 109 meshing with the drum driven gear 105 is fixed to the shift spindle 108 when the shift spindle 108 is rotated the drum driven gear 105 the shift drum 103 and the shift cam 106 are rotated together by the sector gear 109 the shift spindle 108 is connected through an operating cable not shown to a shift lever not shown provided on the steering handle 14 of the vehicle and is rotated by manually operating the shift lever as shown in fig3 the shift cam 106 is a star plate member and a roller 111 supported at the upper end of a shift drum stopper 110 is in contact with the outer circumference of the shift cam 106 the shift drum stopper 110 is pivotably supported to a in 112 and a spring 113 is engaged with the shift drum stopper 110 to normally bias the roller 111 against the outer circumference of the shift cam 106 this mechanism constitutes a rotational position holding device for the shift drum 103 such that the rotational position of the shift.