PURPOSE: To omit cumbersome adjustment and an attenuator and to make it possible to perform the detection of light receiving levels in a broad range, by connecting a series circuit of a resistor and a power source and a series circuit of a resistor, a diode, and a power source to a photodetector in parallel.

CONSTITUTION: Assume -VB1<-VB2, the bias voltage VB of a photodetector 1 becomes VB1+I0×RB1 when the light receiving level is low and the light current I0 is small; while VB becomes VB2+Vf+I0×RB2 (wherein Vf is a voltage drop in the forward direction of the semiconductor 5) when the receiviing level is high and the current I0 is large. With this regard, when the light receiving level is low, the bias resistance of the photodetector is made large and the degradation in light receiving sensitivity is suppressed. When the light receiving level is high, the bias resistance value is made small, and the degradation in frequency response characteristics can be prevented.