To prevent the warp breakage detector from making mistake in a loom in which a soil-preventing screen is disposed around the periphery of the photoelectronic sensor type warp breakage detector.

A light projector is disposed on one side of the shedding area for the warp yarns T, while a photodetector 22 is set to the other side. A stain- preventing screen 23 is interposed between the shedding area and the warp beams 11 so that the screen 23 may surround the warp beams 11. The stain- preventing screen 23 is made of a black non-reflective sheet material. One face of the stain-preventing screen 23 is the exposed part of the stain-preventing screen 23 that can be seen from near the photodetector 22 and the exposed part plays the role of the reflection inhibition to suppress the reflection of the outer scattering light.