PURPOSE: To improve frequency characteristics and to expand a dynamic range by varying the gain of an open loop of a parallel feedback type amplifier amplifying the output of a photodetector in accordance with an optical input signal level.

CONSTITUTION: A detecting signal of an optical signal L which is detected by a photodetector 4 using an avalanche photodiode (APD) 2 with variable gain as its photodetecting element is outputted to a terminal 10 through a parallel feedback type amplifier 13 and a main amplifier 6. A part of the output signal is inputted to a comparator 8 to which a reference voltage VR is applied through a point value detector 7. In accordance with the size of said output signal, i.e. the level of an input optical signal L, the comparator 8 controls the voltage generated by a high voltage generator 1 and also controls a current amplification factor of an APD2. The output of the comparator 8 controls the gain of the open loop of the amplifier 13 through a comparator 11 to which a reference voltage VE is applied and compensates the deterioration of the frequency characteristics at the reduction of the current amplification factor of the APD2 by the band expansion of the frequency due to the rise of the open loop gain.