An arresting mechanism to prevent a nail gun from cocking and firing when there are no nails in its magazine. A trigger includes a retaining part organized to prevent the trigger from cocking to fire. A cotter is arranged for sliding engagement in the trigger mechanism. The cotter is provided along an axial length with a convex portion. A swingable sensing component is pivotally mounted outwardly of the housing with a magazine end projecting into the magazine and pushed outwardly by nails therein. Consequently the other end of the sensing component is pushed on to and holds the cotter during normal operation so that it does not activate the retaining part. When no nail is in the magazine the magazine end of the sensing component is no longer pushed outwardly so the cotter is moved by a spring to position its convex portion to activate the retaining part so as to jam the trigger whereby it can not cock and shoot.