PURPOSE: To effectively improve the sound quality by generating harmonics in an excellent way over a broad frequency range.

CONSTITUTION: An audio signal to be processed is distributed at four system signal paths from an input terminal 10. The signal after no-processing is fed to a mixer circuit 12B in the 1st signal path. A component of 800Hz-3kHz is extracted by a BPF 14 in the 2nd signal path and a harmonic signal is obtained by differentiation by a differentiation circuit 20A of a harmonic generator 20 and detection by a detection circuit 20B. A harmonic is obtained from a signal whose frequency is 3-6kHz by a BPF 16 and a harmonic generator 22 in the 3rd signal path. A harmonic is obtained from a signal whose frequency is 6kHz or over by an HPF 18 and a harmonic generator 24 in the 4th signal path. These harmonic signals are mixed by a mixer circuit 12A and added to the initial audio signal by a mixing circuit 12B.