17. A speech synthesis system comprising: a dialog information database which manages an entire dialog between a user and a system, and stores information and a dialog history required to proceed with the dialog based on speech acts and intention; a system speaking style determination unit which, by referring to the dialog information database, determines a system speaking style based on a user utterance; a dialog prosody generation unit which, when the system speaking style is determined as dialog speech, generates dialog prosody information including an utterance boundary level indicating a duration of a silent period between each semantic unit by referring to the dialog information database and reflecting discourse information between a user and the system, wherein the utterance boundary level is adjusted based on closeness between semantic units, which is determined by syntax and case; and a synthesis unit which synthesizes a system utterance based on the generated dialog prosody information using by at least one computer system, wherein the speech act provides a speech act classification of the user utterance, so that even when speech acts are identical, the synthesized system utterance varies according to the speech act classification of the user utterance.
18. The system of claim 17 , further comprising a system utterance generation unit which, when the system speaking style is determined as read speech, generates a system utterance corresponding to the user utterance by referring to the dialog information database and provides the generated system utterance to the synthesis unit.