PURPOSE: To precisely detect the position of a welding part by detecting the difference of brightness between the welding part and the parent metal parts arranged on both the sides of the welding part by using an image sensor.

CONSTITUTION: A welded tube P is irradiated by the light projected from two projectors 14, 14 at its welding part W and the parent metal parts on both the sides of the welding part W over a required length in the peripheral direction. The bead of the welding part W of the welded tube P is cut out by a bead cutter under about 1,200°C high temperature, so that the surface of the cut part of the welding part W is darkened by oxidation. The degree of oxidation of the welding part W is larger than that of the parent metal parts on both the sides of the welding part W and the light from the projectors 14, 14 is more reflected by the parent metal parts than the welding part, so that the welding part and the parent metal parts lock dark and bright respectively in appearance. A condensing lens 16 forms the image of the distribution of brightness on an image sensor 15, which converts the image into a signal to detect the position of the welding part W.