1. A speech translation apparatus comprising: a speech recognition unit configured to convert a speech of a first language to a source sentence of the first language by recognizing the speech using a speech recognition dictionary, the speech recognition dictionary storing words of the first language and pronunciation candidates corresponding to the words, and words of the second language and pronunciation candidates corresponding to the words; a translation unit configured to convert the source sentence to a translation sentence of the second language by using a translation dictionary, the translation dictionary storing words of the first language and translated words of the second language corresponding to the words; an unknown word detection unit configured to detect an unknown word of the second language from the translation sentence by using the speech recognition dictionary, the unknown word being unregistered word not stored in the speech recognition dictionary; a pronunciation estimation unit configured to estimate a first pronunciation candidate of the unknown word from a character string of the unknown word included in the translation sentence by using Text-To-Speech technique, and to estimate a second pronunciation candidate of the unknown word from a pronunciation of an original word included in the source sentence, the original word corresponding to the unknown word; and a dictionary update unit configured to register the unknown word, the first pronunciation candidate and the second pronunciation candidate, into the speech recognition dictionary correspondingly, wherein the speech recognition unit recognizes a next speech by using the speech recognition dictionary updated by the dictionary update unit.