A transmission mechanism obtains reduced cost by facilitating a process for guide grooves formed on an inner cylinder of a ball spline mechanism and regulates a movement of a drive sprocket in an axial direction by means of a simple structure. A one-way clutch transmits torque only when a crankshaft rotates positively to a drive sprocket through a ball spline mechanism. A clutch outer body and an inner slide member of the ball spline mechanism are composed of members separate from and coupled to each other by coupling pins to be integrally rotatable in an overlapping portions in an axial direction. Guide grooves for housing balls of the ball spline mechanism are formed on the inner slide member while opening to the axial direction at both ends thereof in the axial direction. The overlapping portion of the clutch is overlapping on the guide grooves in the axial direction, and restricts an outer slide member of the ball spline mechanism from moving in the axial direction beyond a predetermined range of movement with respect to the inner slide member of the ball spline mechanism.