5. A speech synthesis method for adjusting a speech style, comprising the steps of: (a) receiving a sentence with a marked friendliness level; (b) selecting a prosodic model based on the marked friendliness level of the sentence; and (c) generating a synthesized speech of the sentence with the marked friendliness level by obtaining speech segments from a synthesis unit database on the basis of the selected prosodic model, the synthesis unit database storing speech segments for each friendliness level wherein the selected prosodic model includes information of speech act and sentence type that comprises an “opening” speech act and sentence type, a “request-information” speech act and sentence type, a “give-information” speech act and sentence type, a “request-action” speech act and sentence type, and a “closing” speech act and sentence type.
6. The speech synthesis method according to claim 5 , wherein the synthesis unit database stores sentence data and the corresponding speech segments recorded according to each friendliness level, the sentence data including information of speech act, a sentence type, or a sentence final verbal-ending or a combination thereof according to each friendliness level.