1. 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 obtained 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; and a fourth step wherein said codevectors obtained in said third step are spectrum-synthesized to obtain a wideband speech signal.