In bidirectional optical burst transmission using a single optical fiber, a signal is reliably discriminated even if reflected light is contained in a received optical signal. According to a first aspect of the invention, the received signal converted into an electrical signal is compared with a prescribed reference voltage, thereby generating a signal light identification voltage output for identifying the beginning of the signal light. According to a second aspect of the invention, a peak value and a bottom value of the received signal are held with respectively prescribed time constants, and a voltage intermediate between them is generated as a voltage output used to discriminate the signal light. According to a third object of the invention, the waveform only of the reflected light is detected and stored, and a signal light voltage output with the reflected light suppressed is generated by subtracting the stored waveform from the received signal.