1. An information search support device which performs clustering on search-target information, displays information indicating a feature of each of clusters, narrows down information to be selected by performing further clustering on information included in a cluster selected by a user from among the displayed clusters, and supports information search by a user, said information search support device comprising: a cluster selection history information accumulation unit configured to accumulate content information of each cluster and cluster selection history information indicating a cluster selected by the user; a degree of certainty calculation unit configured to calculate a degree of certainty indicating a degree to which a vague search target of the user has been identified, based on the content information of the cluster and 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, search-omitted information that is information included in the cluster which is not selected by the user among the displayed clusters and that is 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 add, to the cluster selected by the user, the search-omitted information extracted by said search-omitted information extraction unit, in the case where the degree of certainty calculated by said degree of certainty calculation unit is equal to or higher than a predetermined threshold.