To further improve prevention measures of heavy fire in a lock device of a firearm.

When a trigger 8 is operated, a latch 7 is rotated to operate a striker holder 6 and a hammer 2 is released. A pendulum weight 44 is swung by repercussion in firing. When the pendulum weight 44 is swung and tilted frontward or rearward from a vertical position, a swinging piece 31 is moved rearward to prevent a nose part 35 at a tip of the swinging piece from entering under a projection 18 extending behind the latch 7. Thereby, while the pendulum weight 44 is being tilted by the repercussion, even if the trigger 8 is operated, its movement is not transmitted to the latch 7, and the next firing can not be made. When the pendulum weight 44 is restored to the vertical position, the nose part 35 enters under the projection 18 and the operation of the trigger 8 is transmitted to the latch 7.