PURPOSE: To always execute the satisfactory underwater communication by delaying the same signal as the signal received from a transmitting side for a certain time, re-sending it from a receiving side and turning the wavefront of a transmitting/receiving waves so that the correlation between the transmitting and receiving signals obtained by the transmitting side can be maximum.

CONSTITUTION: The output of a transmitting signal generator 1a is amplified 2a, sent to a correlation computing element 8a, electro-acoustically converted with an echo sounder/transducer 3a and echo-sounder-transmitted 4 into water. A beam 4 received by an echo sounder/transducer 3b is inputted through an amplifier 6b to a correlation computing element 8b, delaying-processed 12b and inputted to the computing element 8b. The delay-processed 12 signal is made into a return beam 5 and echo-sounder-transmitted 3b. An echo sounder received beam 5 is inputted to the computing element 8a. The rotation angle of echo sounder transducer driving devices 9a and 9b is detected 10a and 10b, echo sounder/transducers 3a and 3b are driven and the radiating surface is changed. The same processing is executed concerning a transmitting signal generator 1b. Thus, even underwater where the environmental change is violent, always satisfactory mutual communications are secured.