PURPOSE: To absorb the disturbance of waveforms at the front and rear edges due to the fluttering, etc. of a slip and to improve the reliability of a slip detecting circuit, by driving with pulses the light emitting element of a device which detects optically a slip and providing a latch circuit consisting of an FF to an output circuit.

CONSTITUTION: A transistor TRQ1 s switched by a pulse signal T1 via an inverter I, and a light emitting element D1 is driven by pulses. An optical pulse train undergoes a photoelectric conversion with a photodetecting element Q2 and then amplification to be turned into a logical signal A. This signal A is supplied to a terminal D of a D type FF. The FF is previously reset R1 when the signal T1 is set at O and supplies the signal T1 to a terminal PC. The output B of the FF is set at 1 by the rear edge of a pulse T1 and a signal A having a slight delay. This state is held when no slip P exists. When the slip P arrives and cut off the ight, signals A and B are set at 0. When the slip P passed through, the signals A and B are set at 1 with the timing of the signal T1. This state is held. This absorbs the disturbance of waveforms due to the fluttering of the slip P and improves the reliability of a slip detecting circuit.