In a multi-layer welding, weld line and a gap width of workpieces (A, B) to be welded are detected by a laser sensor mounted on a robot, during a welding for a first layer, whereby, a welding torch mounted on the robot is made to follow the weld line and welding conditions are adjusted in accordance with the detected gap width. The detected gap width is stored in the storage means together with the weld line data. Then, weldings for a second and subsequent layers are performed by using the stored data in such a manner that the welding torch is made to follow the weld line, and the welding conditions are adjusted in accordance with the gap width.