PURPOSE: To attain optical fiber multiple access communication with high reliability, by duplicatedly providing 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 in the same fiber and transmit it between the repeaters. Normally, one repeater, e.g. 41, is operative, and the terminals 31∼35 are mutually communicated via the external optical fiber transmission line 11. If the repeater 41 is failed, the repeater 42 is operated and the communication between the terminals can be assured by using the internal transmission line 12. If the transmission lines 11, 12 are cut off between the terminals, the transmission between the repeaters 41, 42 is done and the communication between the terminals can be ensured for all of terminals.