To provide the method and the device for acoustic signal processing in which the compatibility is good at a threshold value and the transformation is properly conducted.

An acoustic signal processor 3 is constituted of an arithmetic part 3b which computes the transformation equation for a compression and a compression section 3a which compresses the amplitude values of the acoustic signals. Then, in the section 3a, a certain threshold value Vth is determined against the amplitude values of the inputted acoustic signals. If the value Vth is 1 and the amplitude is less than 1, the value remains as is. If the amplitude is larger than 1, the transformation is made by a smoothly increasing function f(x) and the amplitudes of the acoustic signals are compressed. Moreover, in the section 3b, the transformation equation for the value greater than the threshold values, in which the inclination of the joined part becomes 1, is computed.