To reduce a loss by narrowing the width of a weft yarn detector and shortening the required length of a weft yarn flown each time.

This weft yarn detector 1 of an air jet loom is obtained by forming a protruding part 110 on the upper side of a body 100 so as to contract as compared with the lower side of the body 100 in the detector 1 attached to a slay sward with a bolt and a nut, but integrally housing a light emitting element 120 and a photodetector element 130 for detecting a weft yarn by the projection and incidence of light in the top of the protruding part 110 and hermetically sealing the light emitting element 120 and the photodetector element 130 with a cover 140 of a transparent or a translucent body on the outside of the light emitting element 120 and the photodetector element 130 in the air jet loom comprising the weft yarn detector 1 installed at a position mutually opposite to a tunnel reed so as to detect the weft yarn flown by an air jetting force according to the tunnel reed of the loom.