PURPOSE: To simplify a circuit and reduce a scale by performing violation with a designating pulse in a transmission side in an auxiliary signal transmission system using CMI code.

CONSTITUTION: A main signal 1 inputted to a transmitting part J is changed to a CMI code string, and meanwhile, an auxiliary signal 2 is sampled, and a violation designating pulse 12 is generated by a sampling output 11, and a transmission line signal 13 is transmitted after performing violation for the CMI code string of the main signal. In a receiving part K, the transmission line signal 13 is sent to not only a violation detecting circuit P but also to a CMI decoding circuit O, and it is restored to the original main signal 6. The detecting circuit P detects the violation of the code string 13, and its output 14 is restored to an original auxiliary signal 7 by an auxiliary signal decoding circuit Q.