To provide an apparatus for welding a weld nut with which the spatters to a screw hole of the weld nut can be prevented from sticking without displacement of the weld nut.

During pressurizing a work W2 and the weld nut W1 with an upper side electrode 41 and a lower side electrode 45 faced to this upper side electrode, an upper side electrode air is supplied toward the inner part of the screw hole h1 in the weld nut W1 from a hollow part 44 in the upper side electrode 41. In this way, the upper side electrode air is exhausted to the outer part through a long groove formed in the peripheral wall axial direction of a tapered part 52 of a guide pin 50. Therefore, since the spatters can spatter to the outer part with the upper side electrode air, the adhesion of the spatters onto the screw hole h1 in the weld nut W1 can be prevented.