To reduce power consumption in a submarine station in an underwater data communication equipment.

The transmitter/receiver 11 of a marine station 1 converts an observation signal S5 from the submarine station 2 into a reception signal S1 being an electric signal. An SN ratio measuring instrument 14 measures the SN ratio of the reception signal and transmits SN ratio data S2 to the transmitter/receiver 13. The transmitter/receiver 13 converts data S2 into a transmission signal S3. The transmission signal S3 passes through a switch 12 and it is transmitted to the submarine station 2 as a control signal S4 from the transmitter/receiver 11. The transmitter/receiver 21 of the submarine station 2 converts the control signal 4 into a reception signal S6 and transmits it to a transmitter/receiver 23. The transmitter/receiver 23 reproduces SN ratio data S9 from the reception signal S6 and transmits it to a transmission power setting unit 24. The transmission power setting unit 24 controls the power of the transmission signal S6 which the transmitter/receiver 23 transmits, namely the power of the observation signal S5 which the transmitter/receiver 21 transmits in accordance with the SN ratio data.