PURPOSE: To attain the decision of 1, 0 level without error from a waveform having an amplitude fluctuation in an optical reception signal level due to a modal noise by controlling a threshold voltage level depending on the change in the eye pattern.

CONSTITUTION: If a modal noise takes place, the amplitude of signal is changed rapidly, and since an equalizing amplifier 2 cannot equalize the signal, the peak value is changed as shown in figure in the output. A detection circuit 6 detects the change and inputs the result to a control section 7. The control section 7 changes the threshold voltage Vref in the range of Vref'∼Vref" so as to minimize the distortion of the reproduced output based on the detection signal. Thus, even if the reception signal amplitude has a variation, the threshold voltage is kept to a prescribed voltage relation with respect to the reception signal amplitude so as to minimize the code distortion.