1. A word alignment score computing apparatus for computing a score of word alignment of a bilingual sentence pair of first and second languages, comprising: selecting means, responsive to said bilingual sentence pair and a word alignment for the bilingual sentence pair, for successively selecting words of a sentence in said first language of said bilingual sentence pair in a prescribed order; and score computing means for computing, for every word of the sentence in said first language of said bilingual sentence pair, a score representing a probability of the word selected by said selecting means and a word in said second language aligned with said word by said word alignment forming a correct word pair, and based on this score, for computing a score of said word alignment; wherein in computing a score of a certain word pair, said score computing means computes the score of the certain word pair based on all alignments of words selected by said selecting means preceding that word in said first language which forms the certain word pair.