1. A speech synthesis system wherein synthesis parameters necessary for speech synthesis are input, and a speech segment combination matching said synthesis parameters is selected from a speech segment inventory and concatenated, thereby generating and outputting a speech waveform for said synthesis parameters, comprising: a speech segment storage unit for storing said speech segment; a speech segment selection information storage unit for storing speech segment selection information with respect to a given speech unit sequence including a speech segment combination constituted by speech segment stored in said speech segment storage unit and information regarding appropriateness thereof; a speech segment selection unit for selecting from said speech segment storage unit a speech segment combination that is most appropriate for said synthesis parameters based on speech segment selection information stored in said speech segment selection information storage unit; and a speech synthesis unit for generating and outputting speech waveform data based on a speech segment combination selected by said speech segment selection unit.