PURPOSE: To weld continuously and automatically the outside periphery of a work containing a trough bent corner part, by setting the respective weld lines and welding conditions for a trough bent point from a start point, and an end point from the second start point derived from a trough bent point which has been detected automatically.

CONSTITUTION: A start point P1, a trough bent point P2, and an end point P3 are inputted to a condition setting part 10 through a control panel 8. Subsequently, a welding condition of the first weld lines P1WP2 and the second weld lines P2WP3 is inputted. After the input, when welding is started, a control device 2 starts a welding work in accordance with a prescribed procedure. A trough bent corner detecting part 13 of the control device 2 derives the second start point at a distance of about 0.5W1.5l (l denotes a leg length) from a trough bent point which has been derived from a variation of a welding current, and the second welding is executed by a prescribed procedure. By using a robot having such a function, the outside periphery of a work having a trough bent part can be welded efficiently.