PURPOSE: To provide a code vector selection system which can secure the excellent coded voices even if the code book searching range is limited to a half for the periodical transmission of another 1-bit information in regard of a CODEC which codes the voices with use of a code book.

CONSTITUTION: The generating frequency of the optimum code number is checked up in a code book. If the distribution of the generating frequency is biased, e.g. the code numbers '64'-'127' of a shape code book have high probabilities for selection as the optimum code numbers like an LD-CELP system, a code book of a half compared with the numbers '64'-'127' of the shape code book is searched for limitation of a code book searching range.