PURPOSE: To use a token ring and to secure the reliability of a loop-shaped local area network LAN equivalent to a doubling loop by changing over a transmission line so that a transmission line changing-over circuit can be looped back to the token ring at the time of the occurrence of the defect of the transmission line.

CONSTITUTION: In a loop-shaped LAN connecting device where a transmission line is doubled, respective of transmission lines, which is equipped with a transmission line changing-over circuit 9 and doubled, are used as a token ring and a time division multiplexing loop, and at the time of the occurrence of the defect of the transmission line, the transmission line is changed over so as to loop back to the token ring. Thus, bothe services of the token ring and the time division multiplexing loop can be supplied, and at the time of the occurrence of the defect of the transmission line, the token ring is used and the loop-shaped LAN reliabtlity equivalent to the doubling loop can be secured.