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 to recognize the first language and a second language; a translation unit configured to convert the source sentence to a translation sentence of the second language; an unknown word detection unit configured to detect an unknown word included in the translation sentence, the unknown word being not stored in the speech recognition dictionary; a pronunciation estimation unit configured to estimate a first pronunciation candidate of the unknown word from a representation of the unknown word, and to estimate a second pronunciation candidate of the unknown word from a pronunciation of an original word included in the source sentence 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.