This paper reports on an analysis of vocal irregularity in pitch and amplitude in terms of (1) the "pseudo-phase-portrait," (2) the "fractal dimension" of the fluctuation, and (3) a model generating the fluctuation. A simple mathematical model generating the time series of fluctuation is proposed. Using this model, varying aspects of fluctuations as well as of the pseudo-phase-portraits are successfully described with a single control parameter. We also report that, by the simulation based on the model, a resultant intermittent type of fluctuation can manifest the chaotic behavior.