1. A speech synthesis apparatus comprising: language analysis unit configured to identify a word by performing language analysis on a supplied text; a selection unit configured to select one of first speech synthesis processing of performing rule-based synthesis based on a result of the language analysis and second speech synthesis processing of performing pre-recorded-speech-based synthesis for playing back pre-recorded speech data as speech synthesis processing to be executed for a word of interest which is extracted from the result of the language analysis, wherein said selection unit selects the first or second speech synthesis processing based on a word adjacent to the word of interest; a process execution unit configured to execute the first or second speech synthesis processing, which is selected by said selection unit, for the word of interest; and an output unit configured to output synthetic speech generated by said process execution unit.