A transmission includes a ball spline mechanism having an outer member which can rotate integrally with an inner member by means of balls housed in guide grooves and which can move in the axial direction; and a derailer which shifts a chain wrapped around a first sprocket moving integrally with the outer member from one transmission sprocket to another among a plurality of operating sprockets. The inner member and the outer member include guide surfaces through which the outer member is guided to move in the axial direction through the balls. The outer member is provided with a dust cover for covering the guide surfaces, regardless of the axial position of the first sprocket.