PURPOSE:To efficiently and accurately measure the depth and width dimensions at the optional depth position by obtaining depth information up to dummy members of members to be welded and the groove base and positional information in the groove width direction between the dummy members to be brought into contact with both sides of a narrow groove. CONSTITUTION:When a range finder 1 is arranged above the groove and traversed in the sate with the dummy members 5 brought into contact with both sides 3 of the narrow groove 2, information on the groove sides 3 below from both ends 35 of the groove of the steel surface can be also detected by the range finder 1 via the dummy members. The groove width dimensions at the optional depth position can be then measured by detecting the positions of the dummy member 5. In addition, since properties of the dummy member surfaces are remarkably superior as compared with the groove face, the positions of the dummy members 5 and 6 brought into contact with the groove base 4 and the narrow groove sides 3 are obtained by the range finder 1 as the stable and accurate information and the depth and width dimensions of the narrow groove can be calculated with high accuracy.
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