A favorable high-frequency signal is generated and practical high-frequency signal interpolation is implemented through simple processing. A digital audio signal reproduced by an instrument, which also carries out compression, is supplied as an original signal to an input terminal 1, and this original signal is then supplied to a digital sample and hold circuit 3 via a band-pass filter 2. The signal from the digital sample and hold circuit 3 is supplied to a ±1 multiplier 6, which then alternately inverts sign bits. The harmonic components of this signal in which the sign bits are inverted alternately are extracted by a high-pass filter (HPF) 7. Meanwhile, the original signal from the input terminal 1 is supplied to a delay circuit 8 equivalent to the processing time consumed by the aforementioned digital sample and hold circuit 3 and related circuits, forming an adjusted, delayed signal. The signals from the high-pass filter (HPF) 7 and the delay circuit 8 are then added by an adder 10, and the resulting added signal is then output to an output terminal 11.