1. A data processing method for generating a candidate synonym for an object word used in document data, the data processing method comprising the steps of: generating a first set of candidate synonyms for the object word, based on the whole of the document data; generating at least one second set of candidate synonyms for the object word, based on at least one part of the document data; and narrowing the candidate synonyms contained in the first set using the candidate synonyms contained in the second set, wherein in the narrowing step, determining whether the candidate synonyms in the second set are appropriate synonyms of the object word according to a predetermined criterion, and words matching words in the second set which have not been determined to be the synonyms are removed from the candidate synonyms in the first set unless the words have been determined to be the synonyms within the part in any second set, thereby generating the candidate synonym.