1. A speech synthesis apparatus comprising: a processor configured to function as: a language analysis unit configured to identify a word by performing language analysis on a supplied text; a rule-based synthesis unit configured to perform a rule-based synthesis using a language dictionary for the identified word; a pre-recorded-speech-based synthesis unit configured to perform a pre-recorded-speech-based synthesis using a user dictionary; a calculation unit configured to calculate a waveform distortion between a first synthesized speech obtained by a plyin the rule-based synthesis to a pronunciation registered in the language dictionar and a second s nthesized speech obtained by applying the pre-recorded-speech-based synthesis to pre-recorded speech registered in the user dictionary; a comparison unit configured to compare the calculated waveform distortion with a threshold; and an output unit configured to output the second synthesized speech when the calculated waveform distortion is larger than the threshold, and output the first synthesized speech when the calculated waveform distortion is less than or equal to the threshold, wherein the user dictionar is s articular to a user, and the language dictionary is not particular to the user.