PURPOSE: To keep assuredly an output generating state despite discontinuation of an input signal and without using a memory separately, by connecting a phototransistor to a light emitting diode.

CONSTITUTION: A phototransistor 23 containing a photodiode 23a and a transistor TR23b is connected to a light emitting diode 22 to which the DC voltage E1 is applied via a resistance R1. A phototransistor 26 for output use has the same constitution as the transistor 23. The light irradiated from the diode 22 is received by both photodiodes 23a and 26a. Then the forward bias voltage is applied between terminals 24 and 25 to turn on the TR23b. Thus the diode 22 emits light and then keeps this light emission although application of the bias voltage is discontinued. Therefore the transistor 26 of the output side is also held in a driving state. Thus it is possible to keep assuredly an output generating state despite disconnection of an input signal and without using a memory separately.