PURPOSE: To quickly find out a fault of a transmission line.

CONSTITUTION: A sender side transmission section is provided with a time information addition section 1, which adds a time information signal to data and sends the result, data and the information signal are separated through a time information detection and signal separations section 3 provided in a receiver side reception section and the content of the time information signal is sent to a control section 2. The control section 2 awaits the processing by a designated time till succeeding data comes to the time information addition section 1, and when the time information signal added to a head of the succeeding data is not normally sent, re-transmission is made and discrimination of alarm is given. Thus, intermission of data on its way is not caused and even when a transmission line is physically disconnected, it is detected as a fault.