To provide a preamplifier with which an optical. reception signal can be normally received without the occurrence of waveform distortion even when prebias is contained.

An optical signal is converted to a current signal by a photodiode(PD) 1 and converted/amplified to a voltage signal by a trans- impedance type amplifier circuit 12. When the input signal from the PD 1 is accompanied with the prebias, the offset level of an automatic offset control(AOC) 11 is shifted to timing corrected just for the prebias by a reset signal 6. In the case of containing the prebias more than a fixed amount and in the case of containing no prebias, the offset '0' level of an output signal from the amplifier circuit 12 is different. Therefore, the optimum threshold voltage in each case is set by a first automatic threshold control(ATC) 8 and a second ATC 9 and while receiving a control signal from a prebias detection circuit 7, the outputs of the first and second ATC are selected by a selector 10.