In a data transmission method and apparatus which can perform a switchover of all of the main lines without requiring much construction cost, a transmission apparatus sets, upon frame transmission or reception, combinations of a plurality of main lines with virtual paths established so as to include at least a part of a bandwidth of the main lines in one or more protection lines as redundant configurations. Upon frame transmission, the apparatus copies frames received in the main lines, assigns identifiers of the redundant configurations to the copied frames, and transmits the copied frames to the virtual paths of the redundant configurations. Upon frame reception, the apparatus determines whether the main lines or the virtual paths forming the redundant configurations are valid, deletes the identifiers of the redundant configurations assigned to frames received from the protection lines, and determines whether the frames from which the identifiers have been deleted should be respectively discarded or transferred depending on validity or invalidity of the main lines.