A motor-driven welding gun has a servomotor for displacing a movable electrode tip toward and away from a fixed electrode tip. The servomotor has a motor housing with a copper pipe which is inserted therein when the motor housing is cast. The copper pipe provides a coolant passage in the motor housing for supplying a coolant therethrough. When the coolant passage is supplied with the coolant, the motor housing is forcibly cooled, reducing the amount of heat generated by the servomotor.