1. An amplitude-adaptive vector quantization system comprising: a mean value separating circuit which receives K-dimensional vectors derived from a continuous series of input signals divided into blocks of K-samples and separates out a mean value of said input signal vectors; a predictive differential quantizer which implements DPCM quantization for said separated mean value; an amplitude component calculation circuit which calculates the magnitude of amplitude components of mean separated input vectors; a first amplitude vs. quantization stage correspondence table which determines a quantization state of tree-search vector quantization in correspondence to the magnitude of said calculated amplitude components; and a tree-search vector quantizer which calculates the inner product between each of said mean separated input vectors and normalized output vectors with zero mean arranged to have a tree structure, implements a tree-search for an output vector providing a maximum inner product sequentially up to a last stage, provides a maximum inner product at each stage as an amplitude gain at each stage along with an output vector index, and outputs selectively an amplitude gain of a quantization stage prescribed by said first amplitude vs. quantization stage correspondence table; an index buffer which holds the output vector index of each quantization stage obtained during the quantization of said tree-search vector quantizer; an amplitude gain DPCM quantizer which implements DPCM quantization for said output amplitude gain to provide a DPCM-quantized differential amplitude gain and implements DPCM decoding to provide a reproduction of DPCM-quantized amplitude gain; a second amplitude vs. quantization stage correspondence table which provides a true quantization stage selection signal from said DPCM-decoded reproduction amplitude gain; a coding section which has an index selector for selecting an output vector to be outputted finally from among output vector indices of all stages held in said index buffer, and transmits or records three elements, including said DPCM-quantized mean value, DPCM-quantized amplitude gain and selected output vector, as a coding output; and a decoding section including a mean value DPCM decoder and amplitude gain DPCM decoder which implement DPCM decoding for the DPCM quantized mean value and amplitude gain coded by said coding section respectively, an amplitude vs. quantization stage correspondence table which has the same content as said second amplitude vs. quantization stage correspondence table to determine from the DPCM-decoded amplitude gain a quantization output of which stage of coding section is the output vector index decoded by a vector quantization decoder, said vector quantization decoder implementing decoding for said output vector index by switching output vectors adaptively in response to an output of said amplitude vs. quantization stage correspondence table, an amplitude reproducing circuit which implements amplitude reproduction of said decoded normalized output vector on the basis of said DPCM-decoded amplitude gain, and a mean value adding circuit which adds said DPCM-decoded mean value to said amplitude reproduced vector to produce an output signal vector.