PURPOSE: A fineness sensor, having a thin plate of uniform thickness nipped by disks of uniform thickness with improved accuracy of the planes thereof, and a loosely inserted indicating lever rotatably inserted in one side of the disks without sticking sericine, etc. to the gap nor entering knots therein.

CONSTITUTION: A fineness sensor having a thin plate 13 nipped by hard glass disks (12A) and (12B) having a low specific gravity and a shaft 17 inserted in an assembly of a spring 16, constituted of a fine wire, and applied to an indicating lever 15 and a washer (14A) applied to the one side of the glass disk (12A). A washer (14B) is applied to the outside of the glass disk (12B), and the shaft 17 is threaded and tightened by a nut 19 to fix the washer (14A), glass disks (12A) and (12B), thin plate 13 and washer (14B) on the shaft 17. When a knot 22 reaches the edge of a gap 21, the glasss disks (12A) and (12B) are turned upward, and the knot 22 is raised and separated from the gap 21 without causing a broken yarn. The ends of a new cocoon yarn are turned back at the end feeding point to give a coarse fineness, and the sticking of sericine can be eliminated due to the high frictional resistance in the gap 21.