PURPOSE:To detect securely and easily the position of a weld line and the width of a gap by classifying the weld line of welding works according to the presence or absence of the gap and the presence or absence of face in accordance with the distance data obtd. by using an optical range finder. CONSTITUTION:An optical range finder 1 is provided to a carriage or the like moving along the groove line 2 of works 3, 4, and the distance data Y basing on the flat part of the works is detected at every specified pitch in the X direction orthogonal to the line 2. If max. value Y0=plate thickness Y6, the presence of a gap is judged. If each absolute value = set value YC of the difference near each point of a distance data row, i.e., the respectively max. and min. differential values YS1, YS2 in positions XS1, XS2, the presence of each face is judged and further the width GW of a gap 5 and the position of the weld line are determined. The weld line of the works is thus classified to eight classes according to the presence or absence of the gap and the presence or absence of the face. The position of the weld line and the width of the gap are thus detected with good accuracy.
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