A new method was developed for accurately separating source and articulation filter characteristics of speech. This method is based on the AR-HMM modeling, where the residual waveform is expressed as the output sequence from an HMM. To realize an accurate analysis, a scheme of dividing HMM state was newly introduced. Using the AR-filter parameter values obtained through the analysis, we can construct a vocoder-type formant synthesizer, where the residual waveform is used as the excitation source. Through the listening test on the vowel sounds synthesized using AR-filter from a vowel and excitation waveform from another vowel, it was shown that a "flexible" synthesis with a high controllability on the acoustic parameters were possible by our formant synthesis configuration.