PURPOSE: To make lines in duplicate through the addition or a simple circuit by employing a clock generating circuit for detecting a fault of an optical fiber cable and realizing line changeover with a simple circuit.

CONSTITUTION: First-4th transmission means T1-T4 convert an input signal into an optical signal and reception means R1-R4 convert an optical signal into an electric signal. First-4th optical transmission lines F1-F4 are optical fiber cables and used for sending the optical signal converted by the means T1-T4 to the means R1-R4. First-4th modulation means M1-M4 modulate a data by using a clock and 1st-4th demodulation means D1-D4 demodulate the signal to extract the data and the clock. Clock generating circuits CK1-CK2 generate a clock for detecting a fault. The period of the clock extracted is monitored by 1st and 2nd detection means K1,K2 and when the clock is not normally sent, it is discriminated that it is a fault in the transmission line F1 or F2 and changeover means SW1,SW2 are used to select a standby system.