To solve both a first problem in that a long time is required for picking up an image of a site where a weft yarn 1 passes to the judgment of the presence or absence of the weft yarn 1 from the image data and the shortening of the time is thereby necessary to make a weft yarn detector usable in a loom operating at a high speed when an image sensor 4 is used in the weft yarn detector of the loom and a second problem in that it is difficult to provide the weft yarn detector having a high detection accuracy even in the presence of a jetted liquid when the weft yarn detector using the image sensor 4 is employed in the fluid jetting type loom.

This weft yarn detector of the loom is designed to judge the presence or absence of the weft yarn 1 after summing up or averaging values of the quantity of light for each one transverse row of pixels of the image sensor 4 and reducing the capacity of the image data in order to solve the first problem. Furthermore, the weft yarn detector is designed to sufficiently shorten the accumulation time so that the image is picked up in a separated state of a liquid jetted with the image sensor 4 in the granular form and ensure a part where the weft yarn 1 is not hidden in the liquid in order to solve the second problem. Otherwise, the weft yarn detector is designed to set the accumulation time so that the jetted liquid becomes foggy and is uniformly and thinly taken in the whole image and thereby take only the weft yarn 1 deep.