PURPOSE: To attain the formation of a sharp beam and accurate synchronization holding with a simple means by utilizing parametric transmission/reception as a data communication means and using one wave in primary waves as a timing signal for synchronization detection.

CONSTITUTION: Outputs having frequencies f1, f2 generated from a transmitter 11 are radiated from a sender side 1 via a wave transmitter 12 in the state causing mutual interference underwater. This is the primary wave and secondary waves of f1+f2 and f1-f2 are generated by the nonlinear characteristic of water. The primary waves f1, f2 and the secondary waves f1-f2 and f1+f2 are inputted to a wave receiver 21 at a reception side 2. A BPF 23 transmits only the primary wave f1, supplies it to a sinusoidal wave/rectangular wave converter 25 and a BPF 24 transmits only the modulation signal f1-f2 and supplies it to a demodulator 27. The converter 25 converts the wave f1 into a rectangular wave and supplies it to a frequency divider 26. The frequency divider 25 applies 1/N frequency division to the input rectangular wave and supplies it to the demodulator 27 as a timing signal. The demodulator 27 applies synchronizing detection of a modulation signal from a BPF 2, extracts the information data included in the modulation signal and outputs it as a demodulation signal.