Abstract: A loop transmission system with frame synchronization control includes a loop (1) through which information in the form of a sequence of frames is transmitted, a plurality of node units (51, 52, . . . , 5n; 3, 4) inserted at predetermined positions in the loop (1) for receiving and delivering information, and a supervisory device (2) inserted at a predetermined position in the loop (1) for supervising the transmission of information through the loop (1) in association with the node units (51, 52, . . . , 5n; 3, 4). The supervisory device (2) is operable to deliver a frame synchronization cancellation signal. Each of the node units (51, 52, . . . , 5n; 3, 4) is adapted to detect the frame synchronization cancellation signal and cancel the synchronized operation state in bypassing or taking-in of a node and to restore synchronization in the frame next to the synchronization cancelled frame. Therefore, quick restoration of synchronization after bypassing or taking-in of a node is ensured.