PURPOSE: To detect presence of a yarn in a high reliability by traversing a measuring feeler, which is made a carrier and a saw-tooth like yarn guide part in one body via a pressure sensor, transversely across a warp yarn row.

CONSTITUTION: A measuring feeler M is constructed with a carrier T displaceable to the direction of an arrow A in traversing across a warp K constructing the weaving warp and a saw-tooth like yarn guide part F fitted to the warp via a pressure-sensitive sensor S1. According to the displacement of the measuring feeler M to the direction of the arrow A, a tension is gradually applied to every warps K by a guide side face of the yarn guide part F and tilted to a wedge shape, then after the tension attains at the maximum at the peak of the saw-tooth, the yarn jumps to return to the initial position. The sensor S1 detects the variation of the tension as a weight increase and a weight decrease and an optical sensor S2 located on a side face between the saw-tooth like peak and the yarn face at the initial state of the warp K detects the presence of the warp K jumping and crossing.