PURPOSE:To detect the cut shape of an internal bead in three dimensions and to realize high-accuracy measurement by displacing a detection roller in its height direction while moving the roller slantingly. CONSTITUTION:An internal bead cutter 6 is adjusted to the height of an internal bead position and when the internal bead beings to be cut, the detection roller 11 is made to abut on the weld line of the cut part of the internal bead. Here, a servomotor 17 is rotated forward and backward with an external signal and then a screw shafts 19 rotates forward and backward to make a movable member 20 at right angles to the tube axial direction together with an arm 12, so that the detection roller 11 moves laterally around a shaft 14 as an axis of rotation. Consequently, the detection roller 11 moves slantingly at the internal bead cut part about the weld line because of the relative relation between the moving seam welded tube and the stationary bead cutter 6 and the current moving position of the detection roller 11 is detected by a lateral movement position detecting means 25.
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