PURPOSE: To avoid a deterioration of characteristics caused by a fluctuation of DC level and thus to cope with a discontinuous signal as well as increase the receivable range of the optical input power, by performing a discrimination with a balanced code which is obtained by differentiating a binary pulse signal.

CONSTITUTION: An optical reception signal 1 received of a binary intensity modulation is converted into an electric signal by an optical detector 11, amplified up to such a level as to cause no saturation through a preamplifier circuit 21 and differentiated by a differentiating circuit having a time constant smaller than the pulse duration of a reception signal to be converted into a bipolar balanced signal. This differentiated signal is amplified again by a main amplifier circuit 41 to be supplied to a discriminating circuit 51 having a hysteresis. The circuit 51 works like a Schmitt trigger, and accordingly a binary signal is reproduced.