PURPOSE: To keep an amount of a yarn reserve on a drum constant by discriminating a signal started from an advance of winding in a yarn reserving from signals by passing of yarns drawn from the reserve among the signals from photoelectric means.

CONSTITUTION: This yarn feeder forms a reserve 4 composed of a rotatable winding arm 2 on a fixed drum 1 and signals 31 and 32 are generated in photoelectric tubes 11 and 12 by lights reflected from a reflecting element 16, then a rotation of the winding arm 2 is detected by a sensor 2A and an element 2B. The signal generated from advance of winding of the yarn reserve is discriminated from the signals generated from the passing of the yarn drawn from the reserve and combined with the signal generated by a rotation of the winding arm to decide a speed of a motor 6.