1. A word alignment apparatus comprising: a word extracting portion that extracts each word from an example sentence and from a translation sentence thereof; an alignment calculator that calculates at least one of a similarity degree and an association degree between a word in a first language and that in a second language to perform an alignment between words respectively included in the example sentence in the first language and those included in the translation sentence thereof in the second language on the basis of a calculated value; and an optimization portion that optimizes the alignment by performing a bipartite graph matching.