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 a processor configured to: responsive to said bilingual sentence pair and a word alignment for the bilingual sentence pair, successively select words of a sentence in said first language of said bilingual sentence pair in a prescribed order; and compute, for every word of the sentence in said first language of said bilingual sentence pair, a score representing a probability of the selected word 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 processor computes the score of the certain word pair based on all alignments of the selected words preceding that word in said first language which forms the certain word pair; utilize a recurrent neural network having a first input receiving a word selected by said processor and a second input receiving a word in said second language aligned with said word by said word alignment, and apply the selected word and the word aligned with the word by said word alignment to said first and second inputs, respectively; wherein said recurrent neural network includes: an input layer having said first and second inputs, and computing and outputting word embedding vectors from words respectively applied to said first and second inputs, a hidden layer connected to receive outputs of said input layer for generating, by a predetermined non-linear operation, a vector representing a relation between two outputs from said input layer, and an output layer computing and outputting said score based on the output of said hidden layer; and wherein the output of said hidden lever is applied as an input to said hidden layer when a next word pair is given to said word alignment score computing apparatus.