PURPOSE: To always perform distinct underwater communication by automatically switching an upper transmitter-receiver and a lower transmitter-receiver and automatically selecting and using the lower transmitter-receiver when a submergible body is on the surface of sea water and automatically selecting and using the upper transmitter-receiver when the submergible body submerges.

CONSTITUTION: When a power source is turned on, an output 16a of a reception decider 16 is automatically initialized so that code '0' is generated, and a lower transmitter- receiver 12 is selected. Consequently, when a submergible body 21 on which this submarine communication machine is mounted submerges, an upper transmitter-receiver 11 goes underwater and the signal transmitted from the lower transmitter-receiver 12 is received by the upper transmitter-receiver 11, and code '1' is outputted from the reception decider 16. As the result, a selector 10 is switched to connect the upper transmitter-receiver 11 to a transmission/reception coupler 9. In this state, the reception electric signal of the lower transmitter receiver 12 is inputted to the reception decider 16 through the selector 10, a reception detecting amplifier 14, and a band-pass filter 16. The reception decider 16 outputs code '1' when obtaining correlations between a signal 15a and a transmission electric signal 7a, but it outputs code '1' when it cannot obtain correlations.