1. A method for generating a record sentence to establish a speech corpus, comprising: generating a synthesized sentence of speech and synthesis information related to speech synthesis by performing speech synthesis for a predetermined sentence of text using candidate synthesis units transmitted from synthesis database; selecting an unseen sentence including an unseen unit according to the synthesis information; generating a weight indicating a recording priority of the unseen unit included in the selected unseen sentence; generating a record sentence by combining the unseen unit with the speech synthesis information according to the generated weight; and updating the speech corpus by storing the record sentence including the unseen unit, wherein the synthesis database is updated based on the updated speech corpus, wherein the unseen unit is selected as a synthesis unit when a speech unit of satisfactory quality cannot be obtained from candidate synthesis units extracted from the synthesis database, and is updated based on the updated synthesis database.