1. A voice quality conversion device that converts voice quality of an input speech using information corresponding to the input speech, said voice quality conversion device comprising: a target vowel vocal tract information hold unit configured to hold target vowel vocal tract information that is vocal tract information of each vowel and that indicates target voice quality; a vowel conversion unit configured to (i) receive vocal tract information with phoneme boundary information which is vocal tract information that corresponds to the input speech and that is added with information of (1) a phoneme in the input speech and (2) a duration of the phoneme, (ii) approximate a temporal change of vocal tract information of a vowel included in the vocal tract information with phoneme boundary information applying a first function, (iii) approximate a temporal change of vocal tract information that is regarding a same vowel as the vowel and that is held in said target vowel vocal tract information hold unit applying a second function, (iv) calculate a third function by combining the first function with the second function, and (v) convert the vocal tract information of the vowel applying the third function; and a synthesis unit configured to synthesize a speech using the vocal tract information converted for the vowel by said vowel conversion unit.