PURPOSE: To moderately bring an inclined part and a horizontal part of a car body floor panel to spot welding by providing a pair of robot arms which are horizontal and parallel to each other and can execute a double arm coordination operation on a vertical column of a positioning jig side part, and providing separately a turning stud gun on its tip.

CONSTITUTION: A car body floor panel P is positioned in a positioning jig 59. Subsequently, by placing the car body floor panel P between by a motion in the directions of Y, β, Z1, Z2, X1 and X2 of a pair of robot arms 9, 10, the robot arms 9,10 are positioned in its upper and lower parts. Also, the attitude is corrected so that electrodes 17, 18 of the tip are opposed on a face vertical axis against an inclined part W. Next, the inclined part W is brought to spot welding by pressurizing it by each pressure cylinder 19, 20 and bringing it to electric conduction. On the other hand, in the case of welding of other horizontal part than the inclined part W, as well, it can be welded moderately by changing the attitude of the robot arms 9, 10 and stud guns 15, 16 in the same way as the foregoing.