9. The method of claim 7 further comprising a ninth step wherein the power of said lower-frequency components extracted in said fifth step is increased to a level corresponding to the power of said narrowband signal before being supplied to said eighth step, and a tenth step wherein the power of said high-frequency speech signal obtained in said sixth step is adjusted in accordance with the power of said input narrowband speech signal.
7. A wideband speech signal reconstruction method comprising: a first step wherein an input narrowband speech signal is spectrum-analyzed; a second step wherein the spectrum-analyzed results in said first step are vector-quantized using a narrowband speech signal codebook; a third step wherein the quantized values obtained in said second step are decoded to codevectors, using a wideband speech signal codebook; a fourth step wherein the codevectors decoded in said third step are spectrum-synthesized to a wideband speech signal; a fifth step wherein frequency components lower than the band of said input narrowband speech signal are extracted from said wideband speech signal obtained in said fourth step; a sixth step wherein said quantized values obtained in said second step are decoded to obtain a high-frequency speech signal, using a representative waveform codebook of a high-frequency speech signal higher than the band of said input narrowband speech signal; a seventh step wherein said input narrowband speech signal is up-sampled to compute sample values; and an eighth step wherein said lower-frequency components obtained in said fifth step, said high-frequency speech signal obtained in said sixth step and said sample values computed in said seventh step are added together to obtain a wideband speech signal.