PURPOSE: To transmit a data signal without giving interference to the reception of general receivers, without considerable change of transmission equipment, by transmitting a modulation signal of audio frequency of the data signal superimposed on a sub-channel signal of a stereo broadcast of the sum and difference system.

CONSTITUTION: L and R signals from audio signal input terminals 1, 2 are applied to a matrix circuit 3, a difference signal L-R from the circuit 3 and a modulation signal C of a data signal generating circuit 4 are summed at an addition circuit 5, the sum is modulated with a subcarrier wave from a pilot signal generating circuit 7 and a multiplication circuit 8 at a balanced modulation circuit 5, and a sub-channel signal is outputted. This channel signal, a main channel signal and a pilot signal are carried on a main carrier wave and transmitted from a transmitter 11. An FM signal from the transmitter 11 is detected at a detection circuit 22 of a receiver, the main channel and pilot signals are demodulated at stereo and pilot demodulation circuits 23, 25, a data signal is demodulated at a data signal demodulation circuit 26 to transmit the data signal with the slight modification of the transmission equipment.