1. A voice quality conversion system which converts a voice quality of input speech using vocal tract shape information indicating a shape of a vocal tract, the system comprising: a vowel receiving unit configured to receive sounds of plural vowels of different types; an analysis unit configured to analyze the sounds of the plural vowels received by the vowel receiving unit to generate first vocal tract shape information for each type of the vowels; a combination unit configured to combine, for each type of the vowels, the first vocal tract shape information on the type of vowel and the first vocal tract shape information on a different type of vowel to generate second vocal tract shape information on the type of vowel; and a synthesis unit configured to (i) obtain vocal tract shape information and voicing source information on the input speech, (ii) combine vocal tract shape information on a vowel included in the input speech and the second vocal tract shape information on a same type of vowel as the vowel included in the input speech to convert the vocal tract shape information on the input speech, and (iii) generate a synthetic sound using the vocal tract shape information on the input speech resulting from the conversion and the voicing source information on the input speech to convert the voice quality of the input speech.