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.
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. 10. The method of claim 9 wherein said ninth step also decodes said quantized values obtained in said second step to codevectors, using a narrowband representative waveform codebook, spectrum synthesizes said decoded codevectors to obtain a narrowband speech signal, obtains the ratio between the power of said narrowband speech signal and the power of said lower-frequency components obtained in said fifth step, and multiplies the power of said high-frequency speech signal obtained in said sixth step by said ratio.