PURPOSE: To decrease the number of types of the equipments forming a network by using a medium converter (light/electricity converter circuit) of a single type.

CONSTITUTION: The optical single received from a receiver optical cable 101 is converted into the electric signal by a light/ electricity converter circuit 102 and then sent to an electric interface circuit 104 via a 1st synchronizing signal detecting circuit 103. The signal which undergone the retiming through the circuit 104 is converted into a bipolar signal by a unipolarity/bipolarity converter circuit 105 and sent to a transmitter electric cable 106. Meanwhile the electric signal received from a receiver electric cable 107 is converted into a unipolar signal by a bipolarity/unipolarity converting circuit 108 and then sent to an optical interface circuit 110 via a 2nd synchronizing circuit 109. The signal undergone the retiming through the circuit 110 is converted into an optical signal by electricity/light converting circuit 111 of the same type as the circuit 102 and sent to a transmitter optical cable 112.