An electrode structure is provided in which an upper pin having a magnet is built in a cavity inside an upper electrode so as to be vertically movable to allow its front end portion to protrude downward, and a lower pin is arranged so as to face the upper pin and built in a cavity inside a lower electrode so as to be vertically movable to allow its front end portion to protrude upward. In a state where the front end portion of the upper pin protrudes downward, the weld nut inserted onto it is attracted and held by the magnet. In a state where the upper pin is pushed upward by the lower pin, the weld nut comes off the upper pin and drops due to a less attracting force by the magnet, and then fitted onto the lower pin at a position higher than a workpiece.