To provide a controller for C shaped welding gun capable of surely executing rotation-locking of moving arm without excessively pressurizing a work at any gun position and precisely operating.

Two servo motors 1, 21 are arranged to a housing 2 of welding gun, C shaped arm 27 is fixed to the housing, and ball screw mechanisms 7, 8, by which the rotation of an output shaft 3 of one of servo motor 1 is transformed to the reciprocal movement of moving arm 9, are arranged in the housing. Mechanisms 23, 24, by which the rotation of an output shaft 22 of other servo motor 21 is transformed to the reciprocating movement of C shaped arm, are arranged between a wrist 25 of robot and the housing, a rolling guide mechanism 12 consisting of ball and guide groove is arranged between the moving arm, in which a nut 8 of ball screw mechanism is held, and the housing. By simultaneously driving both servo motors and based on an abutting signal of the electrode 26 at the side of C shaped arm on a work 29, other servo motor is stopped.