PURPOSE: To prevent noise and carry out the sensing with a good accuracy by separately arranging both a laser beam projector for sensing weft yarn picked into a warp yarn shedding and its receptor away from a sensing position and integrally installing a reflecting member or a light transmitting member or a gap with a reed.

CONSTITUTION: This sensor for weft yarn of a loom is obtained by arranging both a projector 18 and a receptor 19 separately away from a prescribed position and forming spaces 18a and 19a under an elevated atmospheric pressure in the front of the projector 18 and receptor 19 and preventing noise due to sticking of waterdrops in reflecting a laser beam outputted from the projector 18 with the weft yarn 13 picked in a warp shedding at the prescribed position, receiving the reflected laser beam with the receptor 19 and sensing the presence or absence of the weft yarn 13 in sensing the weft yarn in the water jetting type loom. A reflecting member 20 for reflecting the laser beam from the projector 18 in the direction different from the receptor 19 is constituted by integral installation thereof with a reed 14 so as to bring into contact with a residual yarn part for each beating of the weft yarn 13 and enabling the cleaning of a mirror surface on the reflecting member 20. Otherwise, a light transmitting member or a gap is formed at a prescribed position of the reed 14 so as not to receive the reflected light from dents, etc., other than the weft yarn 13 with the receptor 19. Thereby, noise is prevented.