Abstract: A data transmission system is disclosed in which a plurality of data transmission service units and terminals are connected to a data transmission line to effect, e.g., an in-line or local area network and wherein one of the units is used as an actual system service unit and another unit is used as a backup unit. In the service unit, each one of a plurality of processors included therein monitors whether the other processors are operational and whether the processing results generated are valid, and if an error is detected, the data transmission control operation of the service unit is frozen and the service unit is disconnected from the data transmission line. In the backup unit, when it detects the freezing of the data transmission control operation of the service unit through the data transmission line, it starts the data transmission control operation in place of the service unit. The backup method and apparatus are applicable to a POS system and a multiple computer system.