An information processing device specifies a plurality of cooccurrence words common to first and second words, and determines synonymity or relevancy about the first word and the second word based on cooccurrence indices between the specified cooccurrence words and each of the first and second words. More specifically, the information processing device selects, from among the specified cooccurrence words, one cooccurrence word in which at least one of cooccurrence indices between the specified cooccurrence word and the first and second words does not exceed a preset first threshold, and obtains a difference in cooccurrence index between each selected cooccurrence word and each of the first and second words, if the difference exceeds a preset second threshold, the information processing device determines the first and second words as relevant word candidates, and if the difference does not exceed the second threshold, the information processing device determines them as synonym candidates.