PURPOSE: To provide a data transmission method capable of dealing with multiple data at the same time for one line.

CONSTITUTION: After quaternary synthetic data is obtained by a multiplex processing superposing binary information data of A system on binary information data b of B system and synthesizing them, each quaternary level is endowed with each different frequency signal to perform a frequency modulation for this data. Namely, the highest level is endowed with a frequency F1, the second highest level is endowed with a frequency F2, the third highest level is endowed with a frequency F3 and the lowest level is endowed with a frequency F4. Then, this synthetic data is transmitted by a FSK system for which the signal wave of each frequency is used.