To provide a cable communication system of structure that judges whether a failure is generated in an active transmitter or in an STM line even without stopping the active transmitter to operate a reserve transmitter in a cable transmitter.

For example, when generation of the failure is detected by a cable receiver 200 and notified to the cable transmitter 100, in which whether or not a communication signal is normally transmitted from the active transmitter 110 to a transmission coupler 130 is detected by a signal detector 140. When no communication signal is detected by the signal detector 140, it becomes clear that the failure is generated in the active transmitter 110 and when the communication signal is detected, it becomes clear that the failure is generated in an STM1 line 300.