To make the size of the underwater communication equipment small in size and light in weight by reducing power consumption of a power amplifier thereby miniaturizing a power supply unit small.

A transmission signal converted into an SSB amplitude modulation signal with a voice signal is given to a comparator 73, where the transmission signal is compared with a triangle wave that is outputted from an oscillator and a triangle wave converter 72 and whose frequency is sufficiently higher than the frequency of the transmission signal and converted into a pulse width modulation signal consisting of a pulse train signal with a pulse width corresponding to a level of the transmission signal. The pulse stringth signal is amplified up to a prescribed level at a driver circuit 74 and highly power-amplified at a class D pus-pull power amplifier circuit 75 with an excellent power amplification efficiency. The power-amplified transmission signal is sent to a wave transmitter-receiver via a low pass filter 76 through which only signals of below the frequency band of the transmission signal.