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, wherein the alignment calculator calculates the similarity degree between the words with reference to a word bilingual dictionary and includes at least one of a shape similarity degree and a POS similarity degree; and an optimization portion that optimizes the alignment by performing a bipartite graph matching, wherein if a weight of a branch in the bipartite graph representing a correspondence relation between a word included in the first language as a first node in the bipartite graph, and a word included in the second language as a second node in the bipartite graph is smaller than a threshold value, the words are removed from the optimized alignment and a modified bipartite graph is produced with the remaining words that were not removed from the bipartite graph.