PURPOSE: To detect normally a weld line without depending on a size of a gap of the weld line by providing a temperature gauge between two materials to be welded, detecting a temperature of the direction orthogonal to the weld line, and detecting a position of the weld line by a temperature which is varied discontinuously.

CONSTITUTION: A surface H of the vicinity of a weld line of a material to be welded 1 is heated by a heat source of a laser beam along the weld line A. When a surface temperature of the materials to be welded 1, 2 of the direction orthogonal to the weld line A is detected by a temperature sensor, a temperature falls discontinuously in the weld line A. In this way, a point in which the surface temperature of the materials to be welded is varied discontinuously can be detected as the weld line A.