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.
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. 7. The speech synthesis method according to claim 5 , wherein the step (c) includes the steps of: (c1) extracting the speech segments from the synthesis unit database using prosodic information of the sentence based on the selected prosodic model; and (c2) synthesizing the extracted speech segments.