PURPOSE: To smoothly and surely weld a work with a tank body and prevent the welding defect.

CONSTITUTION: The heights at first, second and third prescribed positions on the work surface are detected by the work sensor 101 arranged at the tip part 100 of a robot and successively, in the position being apart from positions of the work tack-welded with the tank body and the third measured position at the side surface adjacent to the one side surface of the work are detected by the work sensor 101, respectively. Shifting quantity of the work is calculated from these measured positions and the pre-inputted reference coordinate, the work is welded with the tank body.