An audio signal processing apparatus includes a harmonic overtone adder and an equalizer. The harmonic overtone adder includes a high-pass filter for extracting from an audio signal higher than a first predetermined frequency, a filter for extracting a frequency component lower than half a second predetermined frequency, an harmonic overtone generator for generating a frequency-doubled harmonic overtone component from an output from the filter, and a first combining unit for combining the frequency component output from the high-pass filter and the harmonic overtone component output from the harmonic overtone generator. The equalizer includes a level detector for detecting a level of an overtone component contained in an output from the first combining unit, a gain controller for controlling dynamically the level of the harmonic overtone component contained in the output from the first combining unit, and a second combining unit for combining the output from the first combining unit with the harmonic overtone component.