PURPOSE: To improve the quantizing speed by limiting the change of the vector distance between input signal blocks and obtaining an optimum vector out of a subcode book imparted to an output vector preceding by one sample.

CONSTITUTION: The vector X1 of an input signal block is inputted to a signal level limiter 1 and compared with the vector X1-1 preceding by one sample and given from a delay unit 3 in terms of the distance. If the difference of distances is smaller than a prescribed distance, the vector X1' is directly sent to a vector quantizer 2. Meanwhile the vector X1' limited to a level less than the distance (l) is sent to the quantizer 2. The quantizer 2 selects the optimum vector y1 having the minimum distance from the vector X1' out of a subcode book imparted to the vector quantization output y1-1 obtained at the time of the preceding sample and outputs the vector y1. Thus the time required for the quantizing processing is shortened.