To provide a yarn-monitoring apparatus requiring only slight attaching space in the width and capable of setting so as to rapidly and readily operate according to various requests.

This yarn-monitoring apparatus 10 for flat knitting machines has a housing 11, and at least one yarn brake 12, a knot-monitoring device and a yarn breakage sensor are provided in the housing 11 and a sliding piece 18 movable in a notch 17 of housing 11 is provided in order to control the yarn brake. The base of the sliding piece 18 is slid along an elastic strip piece on the elastic strip piece whose one end is supported and the free end of the elastic piece changes force of a spring acting on the yarn brake 12 according to the position of the sliding piece 18.