1 ) A retrieval system to retrieve document data which have a content specified by an inputted retrieval statement among a plurality of document data, the system comprising: a document database that stores said plurality of document data; a concept database that stores a plurality of prespecified concepts using a hierarchical structure in which a first concept including a second concept is a higher layer of the second concept; a document data concept extracting section that extracts document concepts on the basis of keywords contained in said respective document data, the document concepts being said concepts corresponding to the document data; a retrieval statement concept extracting section that extracts a retrieval statement concept on the basis of a keyword contained in said retrieval statement, the retrieval statement concept being said concept corresponding to said retrieval statement; a concept retrieving section that retrieves document data in which said retrieval statement concept is a higher or lower layer of said document concept among said plurality of document data; and a retrieval result output section that outputs said document data retrieved by said concept retrieving section, as said document data containing the content specified by said retrieval statement.