7 so as to be rotatable about the gear shaft 71 further at an outer peripheral edge of the top surface of the final reduction gear 3 is provided a cylindrical shaped engagement protrusion 33 which protrudes in a direction parallel to the gear shaft 71 a tip end portion of the engagement protrusion 33 is extended into the recessed portion 17 of the lock shaft 1 the engagement protrusion 33 makes contact with an inner wall 17 a of the recessed portion 17 to press the lock shaft 1 in its retracting direction against the biasing force of the spring 13 the solenoid 4 as shown in fig5 includes a main body 41 in which an electromagnetic coil and the like are accommodated therein a plunger 42 with a flange portion 42 a formed thereon a spring 43 for biasing the plunger 42 in the protruding direction and a movable plate 44 which is in contact with one end of the spring 43 and fixed to the plunger 42 this movable plate 44 turns on and off a solenoid switch not shown then the plunger 42 is normally placed at a protruding position by the biasing force of the spring 43 but when a current conduction is given to the solenoid 4 the plunger 42 withdraws toward the main body 41 side against the biasing force of the spring 43 the lock shaft switch 8 is a switch for detecting the position of the lock shaft 1 and when the lock shaft 1 is protruded the lock shaft switch 8 is pushed in by a movable piece 81 being positioned at the convex portion 16 a of the.