Abstract: The invention relates to a method for detecting continuously the direction of the weft threads in a moving woven material. A plurality of unit slits (at least three), hereinafter referred to as a "divided slit arrangement," is provided, the plurality of unit slits having a pitch substantially the same as that of the weft threads. The divided slit arrangement is disposed adjacent to the woven material which moves traversely between a light source and at least three unit weft thread detectors. Each detector comprises an alignment slit and a transducer, and is designed so that a moire (generated by light passing through the divided slit arrangement and the weft threads of the woven material), as transmitted through a condensing lens and the alignment slit to the transducer, is converted to an electric signal.