PURPOSE: To sensitize the turning force of a fineness sensor and to make the yarn fineness always constant regardless of the ripening degree of cocoons, by means of a projected screw rod opposite the sensor side.

CONSTITUTION: A turning force adjustor comprising the yarn friction thin plate nip projections 5 and 6 of the two plates facing each other, the turning plate 2 to attach detachably the fineness sensor wherein the thin plates 7 fastened by a screw, the screw rod 10 or hollow plate for adjusting the turning force of the sensor by moving the weights 12 and 13 on the side opposite the plate 2, and a projected smooth rod to transmit the varying turning force by the chain 17.