a rectangular parallelepiped and as a result a cross section cut perpendicular to the lengthwise direction of the first shift key 100 is a parallel tetragon the length of the first shift key 100 in its lengthwise direction is greater than the diameter of the hub axle 10 and when the first shift key 100 is inserted into the groove 12 of the hub axle 10 it protrudes from the outer peripheral surface of the hub axle 10 by a protrusion portion 100a this protrusion portion 100a hits the second clutch as noted above the clutch 30 is energized in the direction of the first shift key 100 by the first energizing spring 13 one end of this first energizing spring 13 is in contact with the lateral surface of the above mentioned large diameter portion 10a of the hub axle 10 and the other end is in contact with a first spring engagement groove 31f provided to the inner peripheral surface on the first end side of the first clutch 31 as a result the clutch 30 moves in the direction of the first shift key 100 until the clutch 30 reaches equilibrium with the operating force via the first shift key 100 and is positioned at the desired speed step as shown in fig3 a second energizing spring 14 is provided around the outer periphery of the hub axle 10 at a position between the first shift key 100 and the second clutch 32 the length of this second energizing spring 14 is set by a spring setting washer such that the resulting set length will be such that when the first shift key.