PURPOSE: To judge objectively correcting capacity by carrying out just once a test by judging the correcting capacity from the difference between the locus of the weld bead obtd. by sensing welding and the theoretically conceivable locus of the weld bead.

CONSTITUTION: A welding torch 1 is held inclined (angle β) with a test piece 3 and is operated in this state to make sensing welding in an arrow direction along a weld line 4. A welding wire 2 projects longer from the torch 1 on the left side of weaving than on the right side and therefore the welding current is smaller on the left side of the weaving than on the right side. A weld line follow-up device shifts the torch 1 toward the left as shown by an arrow L and the locus 5 of the weld bead is obtd. Judgement is thereupon made from the difference between said locus 5 and the theoretically conceivable locus of the weld bead that the correcting capacity of an arc sensor or the weld line follow-up device is good if there is no difference or substantially no difference. Judgement is made that the correcting capacity is not satisfactory if there is conversely the difference.