1. A speech analyzer which analyzes an input speech to extract a vocal tract feature and a sound source feature, said speech analyzer comprising: a vocal tract and sound source separating unit configured to separate the vocal tract feature and the sound source feature from the input speech, based on a speech generation model obtained by modeling a vocal tract system for a speech; a fundamental frequency stability calculating unit configured to calculate a temporal stability of a fundamental frequency of the input speech in the sound source feature, from the sound source feature separated by said vocal tract and sound source separating unit; a stable analyzed period extracting unit configured to extract time information of a stable period of the sound source feature, based on the temporal stability of the fundamental frequency of the input speech in the sound source feature calculated by said fundamental frequency stability calculating unit; and a vocal tract feature interpolation unit configured to interpolate a vocal tract feature which is not included in the stable period of the sound source feature, using a vocal tract feature included in the stable period of the sound source feature extracted by said stable analyzed period extracting unit, from among the vocal tract feature separated by said vocal tract and sound source separating unit.