A resistance spot welder includes: a motor, having a shaft; a cylindrical casing; a hollow guide rod, inserted into the casing, and adapted to be moved linearly for moving a first electrode tip facing a second electrode tip provided at a gun arm; a ball nut, fixed in an inner circumference of the guide rod; a ball screw, fitted through the ball nut; a coupling, including a first fastening portion into which the shaft of the motor is inserted and a second fastening portion into which the ball screw is inserted and which is opposite to the first fastening portion; and a bearing, fitted between an outer circumference of the first fastening portion and an inner circumference of the casing. The guide rod is moved between a first point which is a motor side and a second point which is an electrode tip side. When the guide rod is positioned at the first point, the guide rod accommodates at least a part of the second fastening portion.