PURPOSE: To transmit data most suitably whether the transmission distance is long or shorter, by changing the data transmission speed in accordance with the transmission distance.

CONSTITUTION: A depth sensor 11 detects the depth under the surface of the sea to operate the distance between a ship 2 and a submerged moving body 1, namely, the transmission distance of an acoustic wave signal 3, and the value of this operated transmission distance is stored in a transmission distance acquiring device 12 incorporating a storage means. A transmission speed changing device 14 changes the transmission speed of data 13 in accordance with the value of the transmission distance, for example, by the change of the pulse period of data 13 consisting of a pulse train of plural bit pulses. That is, the pulse period is set to a shorter value if the transmission distance is short. A transmitter 15 and an electro-acoustic transducer 16 constitute a transmission means, and this means converts data of the electric signal consisting of the pulse train of plural bit pulses to an acoustic signal 3 and transmits it. In the ship 2, the acoustic signal 3 is converted to an electric signal by an acousto-optic transducer 21, and data converted to this electric signal is received and processed by a receiver.