PURPOSE: To perform an optical fiber multiple access communication with high reliability, by providing duplicated optical fiber transmission lines in spiral.

CONSTITUTION: Data terminals 31∼35 are connected to duplicated optical fiber transmission lines 11, 12 in spiral via optical repeaters 41, 42 and optical branching couplers 21∼28. The repeaters 41, 42 have functions which convert a transmission wavelength λ1 into a reception wavelength λ2, fold the signal back into the same fiber and transmit it between the repeaters. The terminals 31∼35 normally perform by using the function of either one of the repeaters 41, 42 and if one of the repeaters 41, 42 is damaged, the mutual communication is done by using the other repeater exclusively. If the transmission lines 11, 12 are opened at terminals, the mutual transmission function of the repeaters 41, 42 can ensure the communication between terminals for all of the terminals.