PURPOSE: To attain the use of the optical input IC at a low illuminance by cancelling an output current at a dark state of a 1st photodiode generating a current proportional to an incident light with an output current at a dark state of a 2nd photodiode in a light shut state.

CONSTITUTION: A cathode terminal of a 2nd photodiode connects to an anode terminal of a 1st photodiode 1 and an anode terminal of the 2nd photodiode connects to a cathode terminal of the 1st photodiode 1. Thus, an output current I'o being the sum of an output current of the 2nd photodiode and an output current of the 1st photodiode has a relation of I'o=Id-Id=0 when no incident light exists and the relation of I'o=IL-Id+Id=IL when an incident light exists and the output current Id at a dark state is cancelled. Thus, the linearity at a low illuminance and the temperature characteristic at a low illuminance in the output current Io are improved.