Present invention provides an optical communication system that controls reception sensitivity of an optical receiver. The communication system (100) according to the present invention comprising: an optical transmitter (1) to which an transmission signal is input, and which modulates the transmission signal to an optical signal and transmits the optical signal; and an optical receiver (2) that receives the optical signal and demodulates the optical signal to an transmission signal. And the optical receiver (2) includes a photoelectric conversion means (10) for converting the optical signal into an analog electric signal, a conversion and demodulation means (25) for converting the analog electric signal into a digital signal and demodulating the signal to the transmission signal, and an amplitude control means (102) for controlling amplitude of the analog electric signal, and the amplitude control means (102) controls the amplitude of the analog electric signal in accordance with wavelength dispersion of the optical signal.