PURPOSE: To execute a secure communication by transmitting a code corresponding to a switch input plural times to one switch input.

CONSTITUTION: By operating a switch 1 an encoder circuit 3 is made to be started so as to generate a signal in a specified format. At this time, code information inputted by the input switch 1 is accumulated in a buffer 2 once and then inputted in the encoder 3. The buffer 2 outputs inputted information at intervals of a certain time plural times. Therefore the same code is transmitted plural times. Next, the encoder 3 executes a specified encoding to the signal, which is modulated, for example it is AM modulated, in a modulation circuit 4 and amplified in a power amplifier 5, so that an ultrasonic signal is radiated into the water from an ultrasonic transducer 7 through a circulator 6. Thus a wireless communication between divers, each other, under the water or the one between a diver and a person on the ship can be securely executed.