1. A method performed by a machine translation apparatus which improves a bilingual corpus and which performs statistical machine translation based on the improved bilingual corpus, said bilingual corpus including a plurality of sentence pairs of a first language and a second language and word alignment information of each of the sentence pairs, the method comprising operating the machine translation apparatus to perform functions comprising: extracting a split candidate for splitting a given sentence pair into a plurality of sub-sentence pairs, from word alignment information of the given sentence pair; calculating a split confidence of said split candidate based on both of a number of word alignments across said split candidate and a length of said given sentence pair, wherein said split confidence is calculated such that as a ratio of said number of word alignments across said split candidate to said length of said given sentence pair decreases, said split confidence increases; comparing said split confidence and a pre-set threshold; splitting said given sentence pair at said split candidate in a case in which said split confidence is larger than said pre-set threshold; re-aligning words of sub-sentence pairs split by said splitting to generate an improved bilingual corpus; translating an input sentence from one of the first language and the second language to the other of the first language and the second language, based on translation knowledge trained based on the improved bilingual corpus; and outputting a result of the translating.