A nailing gun includes a swing arm having a first end pivoted to a frame, and a second end movable between a first position proximate to a flywheel to drive a hammer rod to hit a nail, and a second position distal from the flywheel. A spring plate has a spring end portion resiliently supporting the second end of the swing arm. The stop member limits the spring end portion and the second end of the swing arm from moving relative to the frame. When a safety member is subjected to an external pressure, the stop member permits the second end of the swing arm to move together with the spring end portion to the first position. The spring end portion can stably retain the second end of the swing arm at the second position in case of no external pressure.