In this paper, at first, Japanese continuous speech vowel parts PSE (power spectrum envelope) are ob- served, and it is pointed out that there is a great difference between the continuous speech vowel PSE and the.PSE of a vowel uttered in isolation. In order to explain the phenomena, a two-source vocal tract model is proposed. In the model, the vocal tract is expressed as a non-uniform single tube including the under glottal portion. Giving the vowel vocal tract area function corresponding to the 5 vowels of Japanese and two other parameters: the source position and the power ratio, vocal tract pole-zero transfer functions are calculated and an articulatory/acoustic nomogram is constructed. As the results of the inspection if there is a correspondence between the PSE of real speech and one of the nomogram's frequency transfer characteristics in pole-zeroes, the authors present the following hypothesis: "The continuous speech vowel part PSE has the kind of pole-zero structure generated by the two-source vocal tract model".