To detect the position of a groove in a short time by directing a laser beam displace ment meter of a welding torch toward one base metal to store the position coordinates, directing the meter toward other base metal to store the position coordinates, moving the meter to the welding completion position, turning the meter toward one base metal by the prescribed angle to detect the position, and regulating the position coordinates so as to be the previous coordinates.

A work is fitted, and the detection starting command is given. A displacement meter is moved to the waiting position P0, and then, moved to the detection starting position P1 to start the detection. The displacement meter is brought close to a base metal A, and detects the base metal A at the position P2 to store the position coordinates. The displacement meter is turned by the prescribed angle (90°), and brought close to a base metal B, and detects the base metal B at the position P3 to store the position coordinates. The displacement meter is moved in the direction of the welding completion position Pe, and turned toward the base metal A side by the prescribed angle at the position P4 where the distance from the base metal B becomes l2, and the distance l1' from the base metal A is detected. The position of the displacement meter is regulated so that the distance l1 detected at the position P2 is equal to the distance l1'.