PURPOSE: To execute optical transmission stably by a simple circuit constitution without requiring external adjustment by including a peak value holding circuit for inputting an output from a photodetecting circuit, an LPF for inputting the output of the peak value holding circuit and a means for adjusting the bias current of a photodetector on the basis of the output of the LPF.

CONSTITUTION: When an input optical current ip to a pre-amplifier 8 is excessive, the waveform of an output voltage V1 is distorted as shown in Fig.C and the output V1 is turned to an output voltage V2 shown in (b) by the peak value holding circuit 12. The discharge time constant of a capacitor 10 is increased sufficiently larger than the period of an optical pulse signal P and the capacity is selected so that the change of the holding voltage can be neglected at the detection of the rise of the pulse. The output V2 is passed through the LPF17 consisting of a resistor 13(R4), a capacitor 14(C2) and an operational amplifier 15 and having the frequency characteristic of (fc=1/2πR4 C2)-20dB/decade about frequency exceeding fc and outputted as a smoothed voltage V3 regarded as a steady DC voltage for the period of the signal P. The bias current of the photodetector 2 can be adjusted by the voltage V3.