The encoder unit of the vocoder device includes the AR code-book, the MA code-book, and the voice source code-book storing code words each corresponding to a set of the AR, the MA, and the voice source parameters, respectively, which are obtained beforehand by means of the "analysis by synthesis" of a multitude of speech waveform examples and then clustering the resulting respective parameters. The AR preliminary selector, the MA preliminary selector, and the voice source preliminary selector select from respective code-books a predetermined finite number of code words approximating the input speech signal, and in synchronism with the voice source position detected by the voice source position detector the speech synthesizer synthesizes a number of synthesized speech waveforms corresponding to the combinations of the selected AR, MA, and voice source parameters. Comparing the synthesized speech waveforms with the current input speech signal waveform, the optimal code word selector selects the combination of the AR, the MA, and the voice source code words having a minimum distance to the input speech signal waveform.