16. The apparatus of claim 15 further comprising: first power adjusting means for increasing the power of said lower-frequency components at a fixed ratio and supplying the increased power lower-frequency components to said adding means; and second power adjusting means for adjusting the power of said high-frequency speech signal in accordance with the power of said input narrowband speech signal and supplying the power adjusted high-frequency speech signal to said adding means.
15. A wideband speech signal reconstructing apparatus comprising: means for spectrum-analyzing an input narrowband speech signal; means for vector-quantizing the results, obtained by said spectrum-analyzing means, by use of a narrowband speech signal codebook; means for decoding the quantized values, obtained by said vector-quantizing means, to codevectors through use of a wideband speech signal codebook; first speech synthesizing means for spectrum-synthesizing said codevectors, obtained by said decoding means, to obtain a wideband speech signal; filter means for extracting, from said wideband speech signal obtained by said first speech synthesizing means, frequency components lower than the band of said input narrowband speech signal; second speech synthesizing means for decoding said quantized values, obtained by said vector-quantizing means, to obtain a high-frequency speech signal through use of a representative waveform codebook of a high-frequency speech signal higher than the band of said input narrowband speech signal; means for up-sampling said input narrowband speech signal to compute sample values; and means for adding together said lower-frequency components obtained by said filter means, said high-frequency speech signal obtained by said second speech synthesizing means, and said sample values obtained by said up-sampling means, to obtain a wideband speech signal.