In a data transmission system having a plurality of transmitters connected in a ring configuration, a band monitoring unit in each of the plurality of transmitters monitors the bandwidths of a first and a second transmission path between one of the transmitters that comprise the ring and a first and a second transmitter on both sides of said transmitter, respectively. A protection execution unit executes protection processing by setting up a bypass path for each of the first and second transmission paths. Further, when the bandwidth of the first or second transmission path acquired by the band monitoring unit decreases below a threshold, and only when any one of the transmitters comprising the ring and the other transmitters is not executing protection processing, the protection execution unit executes protection processing for the first or second transmission path which is determined to have a bandwidth smaller than the threshold.