A speech analyzer includes a vocal tract and sound source separating unit which separates a vocal tract feature and a sound source feature from an input speech, based on a speech generation model, a fundamental frequency stability calculating unit which calculates a temporal stability of a fundamental frequency of the input speech in the sound source feature, from the separated sound source feature, a stable analyzed period extracting unit which extracts time information of a stable period, based on the temporal stability, and a vocal tract feature interpolation unit which interpolates a vocal tract feature which is not included in the stable period, using a vocal tract feature included in the extracted stable period.