To provide a data transmitting system suitable for a supervisory control system having system redundancy in which cable laying can be altered easily and deterioration in transmission characteristics is eliminated.

Connection is made through a duplex system of a data transmitter 1(#2) one stage ahead of a data transmitter 1(#1) and a data transmitter 1(#3) two stages ahead of the data transmitter 1(#1). The data transmitter 1(#3) performs regeneration relaying for transmitting a signal obtained by summing a data frame obtained by logically summing data frames received from the immediately preceding data transmitter 1(#2) and the data transmitter 1(#1) preceding the data transmitter 1(#2), and a data frame from its own supervisory controller 7(#3). Consequently, a data transmitting system having redundancy for failure of the data transmitter 1 or disconnection of a cable 2 in which deterioration in transmission characteristics is prevented and alteration of cable route is facilitated can be provided.