PURPOSE: To previously prevent a third person from misusing a gun by mounting a stopper on a frame constituting the gun, forming a window hole for rocking the stopper the frame for example, and constituting the stopper rockably.

CONSTITUTION: A protector part 2 of a trigger 3 is formed on a frame 1 constituting a short gun a. Thereupon, there is mounted on the inside of the frame 1 a head of a stopper 6 including on the lower part thereof a concave portion 7 to and through which the trigger 3 is fitted and inserted. A cylinder body 11 is coupled with a mounting part of the head of the stopper 6 and a rotor 10 is fitted and inserted into the inside of the cylinder body 11. Window holes 4 are formed in the frame 1 and the protector part 2 so as for a lower portion of the stopper 6 to undergo a rocking motion. The stopper 6 is rocked by inserting and rotating a key 17 into and in the rotor 10. Hereby, the concave portion 7 is separated away from the trigger 3. Thus, the trigger 3 is locked or unlocked.