PURPOSE: To enable lock and reset operation simply, by providing a stopper projection to lock a stop member and a pushing part which makes contact with an operation piece on the back face of a button of a lock operation, and energizing a lock operation button on one side of the opening part by means of a spring.

CONSTITUTION: A forefinger 40 is hitched on a lock operation button 32, and a middle finger 41 is hitched on a trigger member 25. The finger 40 is moved to slide the button 32, resisting against a spring 36, in the direction of an arrow, and a stopper projection 33 is put on a stopper member 42 and also a pushing part 34 is brought in contact with an operation piece 23. When fingers 40 and 41 are drawn under this condition, the member 25 is operated resisting against a return spring 31. By this operation, the pushing part 34 presses the operation piece 23 and operates a switch body 21, and at the same time, the projection 33 slips on the stopper member 24 to be locked at a stopper hole 24a. Thus the member 25 is locked under its drawn condition. By drawing the button 32, the projection 33 is unlocked from the hole 24a and a drawing force for member 25 is canceled to return the member 25 to its original position by means of the spring force of the spring 31.