PURPOSE: To make the device small-sized and lightweight and reduce the cost, without necessary synchronization by converting the input data signal to an analogue signal and by converting this analogue signal to the FM modulation wave of a frequency corresponding to this analogue signal, in the multiplex data transmission device.

CONSTITUTION: The 6-channel input data signal is converted to an analogue signal corresponding to the arrangement of respective parallel input data signals by D/A converting circuit 7 of transmission part 50 and the frequency is converted to the FM modulation wave of a sine wave or a square wave corresponding to the analogue signal level by voltage-frequency converting circuit 8 and is transmitted. After this FM modulation wave is converted to an analogue signal by frequency- voltage converting circuit 9 in receiving part 60, it is converted to a 6-channel parallel data signal in A/D converting circuit 10 by the clock signal from clock signal generating circuit 11. Consequently, synchronization between the transmission part and the receiving part is unnecessary.