PURPOSE: To improve the quality of an output transmission signal by correcting the current which drives a light emitting element with use of a partial output signal of an operational amplifier and also correcting the current produced by a photoelectric transducer.

CONSTITUTION: A power supply +Bis connected to the emitter of a current control transistor TRTr1, and the collector of the Tr1 is connected to the cathode of a diode D1 for photodetecting element. The output terminal of the D1 is connected to a reverse input terminal of an operational amplifier 4, and a variable resistance terminal connected to the power supply +B is connected to a non- reverse input terminal of the amplifier 4. The output terminal of the amplifier 4 is led to the inverse input terminal of an operational amplifier 6, and the variable resistance terminal connected to the +B is connected to the non-inverse terminal of the amplifier 6. The output of the amplifier 6 is applied to the base of the Tr1 as well as to an LEDD2 via a TRTr2. Then the digital signal is applied to the base of a TRTr3 having its collector connected to the collector of the Tr2. Thus an LED is controlled.