In a quantizer of the invention, a quantization distortion in the case where the optimum vector code word was used and a quantization distortion in the case where the optimum predictive error vector code word was used, are compared by comparing means, thereby selecting which one of the conventional vector quantization and the predictive error vector quantization should be executed for the present input vector. On the other hand, an inverse-quantizer selects which one of the inverse-quantizations should be executed on the basis of the input code number from the vector code. According to the invention, further, the quantizer and inverse-quantizer have the same analyzing means and converting means and execute the adaptation of the code tables by using the quantization results. Thus, the adaptation of the code table of the quantizer and inverse-quantizer can be accomplished without, particularly, adding the information transmitting process for adaptation. On the other hand, by executing the quantization by selectively using the predictive error vector quantizing means, the accuracy of the quantization result is improved and the adaptation of good code tables is realized.