1. An information search support device which supports information search by a user, said information search support device comprising: a display unit configured to perform a first clustering on search-target information, and to display a plurality of clusters and information indicating a feature of each of the displayed clusters; a clustering unit configured to perform a second clustering on a search target of the user, the search target of the user including information included in a cluster selected by the user from among the displayed clusters; a processor; a cluster selection history information accumulation unit configured to accumulate, using said processor, cluster selection history information as content information of each of the displayed clusters at a time of performing the second clustering, the cluster selection history information indicating one of the following: a number of pieces of information included in the cluster selected by the user; a number of clusters selected from among the displayed clusters, and a number of documents including a representative word representing the cluster selected by the user among documents indicating contents of the search-target information; a degree of certainty calculation unit configured to calculate a degree of certainty indicating a degree of clarity for the search target of the user, based on the cluster selection history information accumulated in said cluster selection history information accumulation unit; a search target estimation unit configured to obtain, from content information of the cluster selected by the user, a condition for estimating the search target of the user; a search-omitted information extraction unit configured to extract, from a cluster that is not selected by the user from among the displayed clusters, search-omitted information that is (i) information included in the cluster that is not selected by the user and (ii) information belonging to a set of information estimated from the condition obtained by said search target estimation unit; and a search-omitted information addition unit configured to (i) judge whether or not the degree of certainty calculated by said degree of certainty calculation unit is equal to or higher than a predetermined threshold, and (ii) add, to the cluster selected by the user, the search-omitted information extracted by said search-omitted information extraction unit, in a case where the judgment shows that the degree of certainty calculated by said degree of certainty calculation unit is equal to or higher than the predetermined threshold, wherein said clustering unit is configured to perform a third clustering on the search target of the user after said search-omitted information addition unit adds the search-omitted information to the cluster selected by the user.