Provided is a signal processing method which can enhance the resolution of a spectrum round off by quantization and compensate energy of a spectrum truncated to zero by quantization so as to achieve reproduction without dissatisfaction or uncomfortable feeling. The selecting circuit selects a plurality of coefficients from coefficients of a frequency band of a dequantized acoustic signal. The computing circuit then computes an interpolation coefficient of a coefficient, which is not selected by the selecting circuit, by an interpolation method such as a Lagrange's interpolation method or a spline interpolation method which uses the plurality of coefficients selected by the selecting circuit.