A welding gun driving device is provided with an electric motor, a feed screw mechanism, and a pressure rod axially moved forward and backward via the feed screw mechanism by the electric motor. A rotor of the electric motor is formed into a hollow shape through which the pressure rod is insertable. The feed screw mechanism is provided with a screw shaft fixed to the rotor concentrically with the rotor, and a nut portion screwed to the screw shaft. The pressure rod is provided with the nut portion and a hollow rod portion extending toward the axial front from the nut portion. The rotor is provided, at its inner peripheral face, with a guide portion through which the pressure rod is inserted and supported so as to be relatively rotatable and axially slidable.