PURPOSE: To reduce induction noise due to capacitive coupling infiltrating into the wave receiving face of an oscillator from underwater.

CONSTITUTION: An underwater acoustic sensor 1 is composed of a wave receiving part 20 serially connecting a first oscillator 10 generating positive voltage at the time of adding positive sound pressure (pressure) to the wave receiving face and a second oscillator 11 generating negative voltage at the time of adding negative sound pressure (pressure) to the wave receiving face and outputting an electric signal, and an amplifier circuit 30 amplifying the electric signal outputted from the wave receiving part 20. An acoustic signal subjected to electric conversion by the use of the first and second oscillators 10 and 11 is supplied to the amplifier circuit 30 to be amplified. But the induction noise due to by the capacitive coupling is canceled and not amplified by the amplifier circuit 30. Thus, induction noise is reduced so as to make an SN ratio high.