A log spectral envelope sequence L0, L1, . . . , LN−1 and an envelope code for the log spectral envelope sequence L0, L1, . . . , LN−1 are obtained. The log spectral envelope sequence L0, L1, . . . , LN−1 is an integer value sequence corresponding to binary logarithms of respective sample values of a spectral envelope sequence and is an integer value sequence whose total sum is 0. For a quantized spectral sequence {circumflex over ( )}X0, {circumflex over ( )}X1, . . . , {circumflex over ( )}XN−1, a smoothed spectral sequence ˜X0, ˜X1, . . . , ˜XN−1 is obtained by: for {circumflex over ( )}Xk with Lk being a positive value, adopting {circumflex over ( )}Xk with Lk digits from its least significant digit removed as ˜Xk; for {circumflex over ( )}Xk with Lk being a negative value, adopting {circumflex over ( )}Xk with −Lk digits added to its least significant digit in accordance with a predefined rule as ˜Xk; and when Lk is 0, adopting {circumflex over ( )}Xk as ˜Xk. The respective samples of the smoothed spectral sequence ˜X0, ˜X1, . . . , ˜XN−1 are then encoded with a fixed code length to obtain a signal code.