PURPOSE: To decrease the interference due to surrounding light, by detecting a DC current due to the surrounding light of a photodetector in a reception circuit and changing over the circuit so as to decrease the impedance for signal detection when the DC current reaches a prescribed value or over.

CONSTITUTION: When the brightness of surrounding is not much, a DC current of a photodiode 11 is less and a potential difference across a resistor R1 is less. Thus, transistors (TRs) TR1, TR2 are turned off. Thus, the impedance for signal detection comes from a resistor 21 only. When the brightness of surrounding is increased, the DC current of the photodiode 11 is increased and the potential difference of the R1 is increased. When the brightness reaches a certain value or over, the TRs1,2 are turned on and the signal detection impedance depends on the resistors 21 and R2. Then, the potential at point E is lowered and the potential difference across the photodiode 11 reaches a suitable value.