To stabilize the detection of a weft by uniformizing the received light quantity on a light-receiving face.

This detection apparatus is composed of a supporting means to oppositely position a light projector and a light receptor at both sides of a weft-flying path formed by a deforming reed and a pair of transparent covers 30a, 30b oppositely positioned in front of each of the projector and the receptor interposing the weft-flying path between the covers. The transmission region of the light projected from the projector 32 to the light-receiving face of the receptor 34 is completely enclosed by the covers 30a, 30b and, accordingly, the light is directly transmitted to the light-receiving face without causing the diffusion, and the quantity of received light is uniformized to stabilize the detection of weft.