1. A device for determining interest, comprising: a storage device that stores, 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; a hardware processor configured to: accept, from a person who is to conduct a search, designation of a second document and any one of a plurality of users; and determine 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.