PURPOSE: To quickly connect a data transmission line as normal connection state at system start-up or fault restoration by providing a duplicate loop control section controlling the loopback constitution and release to each data transmitter.

CONSTITUTION: Every time power supply is applied to data transmitters 1-3 at start-up of the system, active and standby system data transmission lines Lm, Ls are constituted to be sequentially loopback constitution to adjacent transmitters 1-3 via a duplicate loop control section CP of each equipment. Then the transmission line is expanded and the condition of loopback constitution is satisfied to adjacent equipments of upstream and downstream, then the loopback constitution is release and the normal loop is constituted. At restoration of fault, the loopback constitution is released quickly by the same procedure as system start-up and normal connection state is caused to the transmission line Lm. That is, the data transmission line is brought into the normal connection state at start-up of the system or fault restoration.