PURPOSE: To attain ease of maintenance of a loop line by allowing an optional data transmission equipment on a loop data transmission system to release the loopback mode of a data transmission equipment used as a maintenance supervisory device.

CONSTITUTION: Line switching sections 400 and 410 conduct the system connection switching to the system "0" or "1" or the loopback via control sections 301, 311 from line control sections 300 and 310. Further, a line switching section 410 checks normally a packet inputted via a loop line 240 or a receiver circuit 241 and informs that a specific packet is recognized to the line control section 310 via the control line 311 when the specific packet is recognized. The line control section 310 outputs a signal to stop the packet transmission to a driver circuit 211 or 231 from the line switching section 400 to the line control section 300 via a control line 500 and transmits the said specific packet to loop lines 210 and 230 via a data 212, the driver circuit 211, a data line 232 and the driver circuit 231.