11. A non-transitory computer-readable storage medium storing thereon a computer program used in a computer, the computer program causing the computer to perform processing comprising: storing, on a user-by-user basis, a co-occurrence frequency in correlation with a user, the co-occurrence frequency indicating how many times a pair of words is used in a same cluster of a first document, on a pair-by-pair basis, to which the user gained access previously; receiving a designation of a second document and any one of a plurality of users; and determining that, among a plurality of pairs of words, a pair which is used in a same cluster of the designated second document and which also satisfies a predetermined condition of the co-occurrence frequency corresponding to the designated user is a particular pair in the second document.
15. The non-transitory computer-readable storage medium according to claim 11 , comprising searching for a document containing all of words making a particular pair.