An optical transmitter generates a transmission signal having a frame as a unit, the frame including a signal pilot signal with fixed amplitude and phase and two data bits to output the generated signal into a transmission line. In a receiver, a splitter splits the signal light from the transmission line. An optical delay delays the signal light for a 1-bit period. A first combiner combines the signal light from the transmission line and the output signal light from the optical delay. A photodetector converts the combined light into an electrical signal. A 3-bit optical delay, a second combiner, a BPF, and an oscillator generate a frame-timing signal. A gate separates a data from the output from the photodetector under the control of a gate control unit. A binary discriminator binary-discriminates the output signal from the gate.