A welding gun precludes moisture from penetrating inside of a motor housing due by preventing development of negative pressure inside the motor housing. The welding gun includes a feed screw mechanism that is coupled to a servo motor and has a hollow rod that reciprocally moves in a predetermined direction by way of torque applied by the servo motor, and a moveable electrode tip that is connected to a leading end portion of the hollow rod and opens and closes relative to a fixed electrode tip according to the reciprocal movement of the hollow rod. The servo motor has a motor housing in which a motor housing hollow part accommodating a portion of the hollow rod to be reciprocally moveable is formed. A vent that ventilates the motor housing hollow part with ambient air is formed in the motor housing.