5. The retrieval system according to claim 1 , wherein said concept retrieving section comprises: a higher concept acquiring section that acquires a retrieval statement higher concept that is a higher-layer concept of said retrieval statement concept if said retrieval statement concept does not match said document concept; and a generalized concept output section that outputs the document data as a retrieval result if said retrieval statement higher concept matches said document concept.
6. The retrieval system according to claim 5 , wherein said concept database stores each of said plurality of concepts as a node of said first or second hierarchical structure, said document data concept extracting section extracts said first document concept belonging to said first hierarchical structure and said second document concept belonging to said second hierarchical structure in association with said document data, said retrieval statement concept extracting section extracts said retrieval statement concept belonging to said first hierarchical structure and said second retrieval statement concept belonging to said second hierarchical structure in association with said retrieval statement, said higher concept acquiring section acquires said first retrieval statement higher concept that is a higher layer of said first retrieval statement concept and said second retrieval statement higher concept that is a higher layer of said second retrieval statement concept if said first retrieval statement concept does not match said first document concept and if said second retrieval statement concept does not match said second document concept, and said generalized concept output section outputs said first document data as a retrieval result if the number of said first document data in which said first retrieval statement higher concept is the same as said second retrieval statement concept and in which said first document concept is the same as said second document concept is smaller than that of said second document data in which said first retrieval statement higher concept is the same as said second retrieval statement concept and in which said first document concept is the same as said second document concept.