First and second coefficients are fed into a Fast Fourier Transform unit through real number input and imaginary number input portions thereof, respectively, to perform the Fast Fourier Transform of the entered first and second coefficients, thereby producing a frequency-domain coefficient vector. The Fast Fourier Transform of an input signal is performed to transform the input signal into a frequency-domain signal vector. Thereafter, the transformed signal vector is multiplied by the coefficient vector for each element, thereby providing a multiplication result. The Inverse Fast Fourier Transform of the multiplication result renders real number output and imaginary number output portions of the inverse transformation result as first and second series of output signals, respectively.