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 hardware processor; a vowel receiving unit configured to receive sounds of plural vowels of different types, each type of the vowels being a representative vowel of a spoken language; an analysis unit configured to analyze, using the hardware processor, 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, wherein the combination unit includes: an average vocal tract information calculation unit configured to calculate a piece of average vocal tract shape information by averaging plural pieces of the first vocal tract shape information generated for respective types of the vowels; and a combined vocal tract information generation unit configured to combine, for each type of the vowels received by the vowel receiving unit, the first vocal tract shape information on the type of vowel and the average vocal tract shape information to generate the second vocal tract shape information on the type of vowel.