12. The machine translation method according to claim 10 , further comprising: translating the first language sentence into the third language sentence when the number of the intermediate second language sentences corresponding to the first language sentence is singular.
10. A machine translation method to translate a first language sentence into a second language sentence, the machine translation method comprising: translating, by a computer, the first language sentence of a first language into an intermediate second language sentence of a second language; acquiring a size of a corpus of a third language; determining whether or not the size of the corpus of the third language is larger than a threshold value; translating the intermediate second language sentence into a third language sentence of the third language when the size of the corpus of the third language is larger than the threshold value; outputting the intermediate second language sentence when the size of the corpus of the third language is less than or equal to the threshold value; searching a database storing the corpus of the third language including more example sentences of the third language than example sentences of the first language and the second language using the third language sentence as a search key to acquire one or more other third language sentences; determining, based on the one or more other third language sentences, a final second language sentence of the second language corresponding to the first language sentence as the second language sentence; calculating a degree of coincidence between the search key and each example sentence of the third language in the search result; calculating an average value of the degrees of coincidence; and setting the average value as the degree of coincidence corresponding to the search key.