An information search support device includes: a cluster selection history information accumulation unit which accumulates content information of each cluster and cluster selection history information indicating a cluster selected by a user; a degree of certainty calculation unit which calculates a degree of certainty indicating a degree to which a vague search target of the user has been identified, based on the cluster selection history information accumulated; a search target estimation unit which obtains a condition for estimating the search target; a search-omitted information extraction unit which extracts search-omitted information that is included in the cluster that is not selected and is estimated from the obtained condition; and a search-omitted information addition unit which adds, to the cluster selected by the user, the extracted search-omitted information, in the case where the degree of certainty is equal to or higher than a predetermined threshold.