A data transmission system for an automotive vehicle includes a plurality of control units installed on the vehicle, and a transmission line connecting the plurality of control units with each other. The transmission line comprises a pair of signal lines to which digital data signals having opposite polarities are delivered for transmission of data. Each of the plurality of control units comprises a first receiver for receiving both of the digital data signals having oppositive polarities and outputting a first difference signal indicative of a difference between the digital data signals, a second receiver for receiving one of the digital data signals and outputting a second difference signal indicative of a difference between the one of the digital data signals and a predetermined reference level set thereto, and a third receiver for receiving the other of the digital data signals and outputting a third difference signal indicative of a difference between the other of the digital data signals and a predetermined reference level set thereto. Detecting of which of the pair of signal lines is faulty, is carried out based on the first difference signal output from the first receiver, the second difference signal output from the second receiver, and the third difference signal output from the third receiver.