PURPOSE: To prevent turning motion of an arm due to the external force, by rotating one of the arms with a gate driving source and rotating the other of the arms while sliding a pin of one arm in a long hole of the other arm.

CONSTITUTION: The first arm 13 and the second arm 15 are formed in such a manner that when the first arm 13 is stopped in a stop position H or L, a pin 12 is adapted to the outermost end of a long hole 14, and the long axis of the long hole 14 crosses at an angle smaller than 90° with the long axis of the first arm 13. Accordingly, if turning force is applied to a gate 23, set in a stop position h or l, to the other stop position, pressing force to the direction of a driving shaft 11a and externally sliding force in the long hole 14 are applied to the pin 12. Here the pin 12 is adapted to the outermost end of the long hole 14, and the pin 12 can not be moved even if pressed externally of the long hole 14.