32 c of the rotary plate 32 reach the horizontal portion 11 f 2 of the engagement groove 11 f as shown in fig9 c the protrusions are rotated rightward as shown by the arrow r by the restoration force of the rotary drive spring 33 twisted by the rotation of the rotary cover 31 and the rotary plate 32 as a result the protrusions 31 c and 32 c move to the right end of the horizontal portion 11 f 2 of the engagement groove 11 f further since the rotary cover 31 is also driven upward by the axial restoration force of the rotary return spring 33 the rotary cover 31 rises and only the protrusion 31 c of the rotary cover 31 moves into the vertical portion 11 f 3 of the engagement groove 11 f as depicted in fig9 d as a result the protrusion 31 c of the rotary cover 31 is locked to the vertical portion 11 f 3 of the engagement groove 11 f of the operation unit 11 therefore the rotary drive portion 30 is fixed and cannot rotate with respect to the body portion 11 a of the operation unit main body 10 the operation unit 10 is thus inserted to the very end into the rotary drive portion 30.