An index update unit analyses the information stored in a document repository to create an index for search and stores the index in a time-series divisional index storage unit and creates, from an ACL repository, an access control entry ACE in association with the index for search, which is correlation of information to be searched with access right of at least a group to which the user belongs. The ACL cache generation unit creates ACL cache data that correlates the user with access right to the information to be searched, from the ACE, and registers the ACL cache data created in an ACL cache. A search processing unit searches for an index for search in response to a request for search from said user. In case the ACL cache data correlating the user with the index for search is registered in the ACL cache, the search processing unit takes, from among the information searched, the information, reference to which is allowed for the user as a search result, based on information in the ACL cache.