1. A speech signal encoding method, in which a vector quantization technique is used to select an optimum code vector from fixed-dimension code vectors stored in a codebook of fixed-dimension code vectors for a variable-dimension input vector, the speech signal encoding method comprising: a step for determining prediction residuals of an input speech signal to be encoded; a step for performing sinusoidal analytic encoding of the prediction residuals of the input speech signal to produce spectral harmonics data; a step for determining a variable-dimension input code vector from the spectral harmonics data; a step for converting the variable-dimension input code vector into a fixed-dimension input code vector; a fixed/variable dimension conversion step for converting a plurality of the fixed-dimension code vectors read out from the codebook of fixed-dimension code vectors into a plurality of variable-dimension code vectors; a selection step for selecting from the codebook of fixed-dimension code vectors an optimum code vector of the plurality of variable-dimension code vectors converted in the fixed/variable dimension conversion step which minimizes an error between the variable-dimension input code vector of the spectral harmonics data of the input speech signal and a fixed dimension code vector; and a step for outputting an encoded speech signal.