PURPOSE: To remarkably easily secure minimum quantized distortion by executing vector quantization with a reference pattern reclustered by clustering permitting duplication.

CONSTITUTION: A ROM 5 is an EPROM for storing reference patterns reclustered by clustering permitting the duplication in the order of addresses corresponding to the execution order of pattern matching/searching of steps 1, 2 and the reference patterns used for the pattern matching are built in the ROM 5, read out under control by a controller 6, stored in a shift register 7, and then sent to a pattern collator built in a processor 1 through an output line 701. The controller 6 controls the operation timing of respective constitutional apparatuses through I/O lines 501, 601, 603 and output lines 602, 604, etc. The pattern matching is executed on the basis of a format for electing a reference pattern having the minimum spectrum distance Das out of the stored reference patterns.