1. A method of constructing translation knowledge, comprising: receiving a source-language sentence and a target-language sentence corresponding to translation of the source-language sentence, which being performed by a constructing translation knowledge apparatus; converting the source-language sentence and the target-language sentence into a form that is expressed with syntactic information, which being performed by the constructing translation knowledge apparatus; performing word alignment and syntactic alignment of the converted source-language sentence and the converted target-language sentence, which being performed by the constructing translation knowledge apparatus; and extracting translation knowledge on a subcategory of a bilingual inflected-word from results of the word alignment and the syntactic alignment by using information on dependency relationship, which being performed by the constructing translation knowledge apparatus, wherein the performing includes: reconstructing the converted source-language sentence and the converted target-language sentence by using at least one of a surface type, a prototype, and a part-of-speech of a morpheme, and generating pairs of a plurality of reconstructed sentences; outputting a plurality of results of the word alignment by searching words corresponding to each other in the source-language sentence and the target-language sentence with respect to the pairs of the plurality of reconstructed sentences; correcting the results of the word alignment by using a previously held bilingual dictionary; post-processing that determines a proper word alignment as a word alignment that is obtained by, with respect to the plurality of results of the word alignment, excluding the corrected word alignment results and taking an intersection set; and performing the syntactic alignment by using the result of the proper word alignment and information on dependency relationship of the source language.