PURPOSE: To prevent confused detection of knot and thread breakage by identifying them through detecting plus or minus of differentiation of the amount of light received.

CONSTITUTION: When thread breakage exists, electromotive force of photodiode 14 increases, pluse puls differentiated by condensor 16 is amplified by amplifier 18 to cause its output voltage V exceeds upper limit Vu, and output 36 of amplifier 29 generates plus pulse which enters input terminal T of flip-flop 38. The status of R=0, D=1 is changed to Q=1, Q'=0 by clock and transistor 42 is thru to indicate thread breakage and to be automatically reset. When knot N is detected, amplifier first generates minus pulse and then plus pulse. Amplifier 30 first gives pulse to flip- flop 39 with its output being Q=1, transistor 43 is thru to generate signal of knot.