PURPOSE: To remove wave receiving input in a specific direction, by using two receiving beams different in a main pole direction and having the same acoustic center and setting the difference between receiving sensitivities of inputs due to two receiving beams relating to a specific direction to zero.

CONSTITUTION: The reverberation from the surface of the sea in a direction of an angle θ1(t) is received by the common receiving cover region of two wave receiving beams 1, 2 and the outputs of beams 1, 2 are supplied to amplifiers 3, 4. The receiving sensitivity difference calculator 51 of a receiving sensitivity difference correction part 5 receives the data relating to θ1(t) from a sonar apparatus main body and calculates the directivity of the beams 1, 2 from the directional data preliminarily stored in internal ROM to supply the same to a sensitivity difference correction device 52. The correction device 52 adds sensitivity difference G(t) to the input based on the receiving beam 2 in the θ1(t) direction received from the amplifier 4 to eliminate the difference with the input of the beam 1 to eliminate the difference with the input of the beam 1 outputted by the amplifier 3. These data are mutually subtracted by a subtractor 53 to bring the sensitivity difference to zero and only sea surface reverberation removing output in the θ1(t) direction is supplied to a signal processor 100.