PURPOSE: To judge existence of abnormality related to weft in high accuracy.

CONSTITUTION: An endless belt 4 is reciprocated and rotated by reciprocating drive of a motor 5 and a sensor head 2 is reciprocated along a rail 1. The motor 5 is rotatably supported on a strut 13. A supporting shaft 6-1 of a guide pulley 6 is held on a supporting bar 14 of a circular arc. A screw part 6-2 at the lower end of the supporting shaft 6-1 is passed through a long hole 14-1. A nut 15 is screwed to the screw part 6-2. The radius center of the long hole 14-1 of a circular arc is on the central shaft line of the strut 13. A rail 1 is hung and supported between a supporting fitment 12 and the supporting shaft 6-1. The rail 1 exists on the radius line of the long hole 14-1 having the central shaft line of the strut 13 as the radius center regardless the fitment position of the screw part 6-2 to the long hole 14-1.