To output signals of high optical S/N with less crosstalks and uniform optical characteristics by multiplexing N optical signals which are outputted from N electrical-to-optical transducers, converting the output of a multiplexing circuit into electric signals and then converting the electric signals outputted from an optical-to-electrical transducer into optical signal.

An optical pulse string outputted from an electrical-to-optical(E/ O) transducer means 2a and an optical pulse string outputted from an E/O transducer means 2b are inputted to an optical multiplexer/demultiplexer 3 with a delay time difference of 50 ps. Thus, the inputted pulse strings are turned into signals 10001111 of 20 Gbit/s. The signal which is outputted from an optical- to-electrical(O/E) transducer means 4 is an electric signal 10001111 of 20 Gbit/s. An E/O transducer means 2c outputs an optical signal 10001111 of 20 Gbit/s. Then an optical signal having a rate twice as high as the signal that is inputted from a data input terminal 1a, etc., is outputted from the E/O transducer means 2c.