An object is to provide an optical modulation/demodulation method, an optical communication system, an optical transmitting device, and an optical receiving device capable of inhibiting an increase in the cost and a decrease in the band at the time of multiplexing services. The optical transmitting device according to the present invention sums a plurality of binary signals that have bit rates having such relation that the bit rate of any higher speed side is twice or more integer multiples of a bit rate of any lower speed side, having smaller amplitude as the corresponding bit rate becomes higher and having matched rise and fall timings, and generates a multi-level signal, and modulates light from one light source. In other words, generating a multi-level signal as a modulation signal enables a plurality of services to be multiplexed by one transmitter. The optical receiving device according to an aspect of the present invention sets a plurality of thresholds that can be used for identifying all the amplitude values of the multi-level signal for an optical signal that is service-multiplexed by the optical transmitting device described above and compares an amplitude of the multi-level signal acquired by performing photoelectric conversion of the received optical signal with the plurality of thresholds. The optical receiving device determines bits of the multiplexed binary signal on the basis of a result of the comparison.