1. A retrieval system for retrieving document data which have a content specified by an inputted retrieval statement among a plurality of document data, the system comprising: a storage device comprising: a document database that stores the plurality of document data; a concept database that stores a plurality of pre-specified concepts using a hierarchical structure in which a first concept including a second concept is a higher layer of the second concept; a concept extraction rule database that stores concept extraction rules comprising sets of one or more of the keywords and concepts indicating meanings of the one or more keywords, a processor configured for: extracting document concepts on the basis of keywords contained in the respective document data, the document concepts being the concepts corresponding to the document data; extracting a retrieval statement concept on the basis of a keyword contained in the retrieval statement, the retrieval statement concept being the concept corresponding to said retrieval statement; retrieving document data in which the retrieval statement concept is a higher or lower layer of a document concept among the plurality of document data; outputting document data retrieved by a concept retrieving section, as the document data containing the content specified by the retrieval statement; wherein the processor extracts the concept contained in the concept extraction rule as the retrieval statement concept if said retrieval statement comprises the one or more keywords contained in any of the concept extraction rules, wherein the processor extracts the concept contained in the concept extraction rule and uses said concept as the document concept if said document data include the one or more keywords contained in any of the concept extraction rules, and wherein the retrieval system further comprises: a retrieval index database that stores, for each of the document data, an association between the document data and the document concept of the document data extracted by the document data concept extracting section, wherein the concept retrieving section outputs said document data corresponding to the document concept of said document concept stored in the retrieval index database before the retrieval statement is inputted; information storage space storing a synonym database that stores an association between a predetermined word or phrase and the keyword that is a synonym of the word or phrase; a processor configured to perform data normalizing section that normalizes the document data by replacing the word or phrase contained in each of said document data with the keyword that is the synonym of the word or phrase; and information storage space storing a retrieval statement normalizing section that normalizes the retrieval statement by replacing the word or phrase contained in said retrieval statement with the keyword that is the synonym of the word or phrase, wherein the processor extracts the document concept from the normalized document data, and the retrieval statement concept extracting section extracts the retrieval statement concept from the normalized retrieval statement; wherein the processor is configured to: acquire a retrieval statement higher concept that is a higher-layer concept of said retrieval statement concept if the retrieval statement concept does not match the document concept; and output the document data as a retrieval result if the retrieval statement higher concept matches the document concept; wherein: the concept database stores each of said plurality of concepts as a node of the first or second hierarchical structure, the processor extracts the first document concept belonging to the first hierarchical structure and the second document concept belonging to the second hierarchical structure in association with the document data, the processor extracts the retrieval statement concept belonging to the first hierarchical structure and the second retrieval statement concept belonging to the second hierarchical structure in association with the retrieval statement, the processor acquires the first retrieval statement higher concept that is a higher layer of the first retrieval statement concept and the second retrieval statement higher concept that is a higher layer of the second retrieval statement concept if the first retrieval statement concept does not match the first document concept and if the second retrieval statement concept does not match the second document concept, and the processor outputs the first document data as a retrieval result if the number of the first document data in which the first retrieval statement higher concept is the same as the second retrieval statement concept and in which the first document concept is the same as the second document concept is smaller than that of the second document data in which the first retrieval statement higher concept is the same as the second retrieval statement concept and in which the first document concept is the same as the second document concept, or wherein the processor is configured to replace the retrieval statement concept with the retrieval statement lower concept: if all the document data having the document concept that is the same as the retrieval statement concept have the document concept that is the same as a retrieval statement lower concept that is a lower layer of the retrieval statement concept; and outputting the document data in which the retrieval statement lower concept matches the document concept, as a retrieval unit; and wherein: the concept database stores the plurality of concepts that identify a plurality of defects in a product, the document database stores the document data indicating contents of each of the plurality of defects, the retrieval statement concept extracting section extracts the retrieval statement concept corresponding to the retrieval statement used to retrieve said defects in the product, and the processor outputs the document data retrieved, as said document data indicating the contents of the defects in the product inputted by a user; or wherein: the concept database stores the plurality of concepts in a lower layer of the concept indicating that there are defects in components of the product, using a hierarchical structure in which the concepts indicating states of the defects in the components are provided, the document data concept extracting section extracts the document concept indicating that there is a defect in one of the components, on the basis of the keyword contained in the document data, the retrieval statement concept extracting section extracts the retrieval statement concept indicating the state of the defect in the one of said components, on the basis of the keyword contained in the retrieval statement, and wherein the concept retrieving section comprises: a higher concept acquiring section that acquires a retrieval statement higher concept that is the concept indicating that there is the defect in the one of said components, the concept being a higher layer of the retrieval statement concept; and a generalized concept outputting section that outputs, as a retrieval result, the document data having the document concept indicating that there is the defect in the one of the components, the document concept matching the retrieval statement higher concept; and further comprising a component database that uses a hierarchical structure to store inclusive relationships among the components of the product, wherein: the processor further extracts the document concept indicating the component described in the document data, on the basis of the keyword contained in the document data, the processor further extracts the retrieval statement concept indicating the component described in the retrieval statement concept extracting section, on the basis of the keyword contained in the retrieval statement, the processor acquires the concept that is a higher layer of the first retrieval statement concept indicating that there is the defect in the component or a state of the defect in the component, and the concept that is a higher layer of the second retrieval statement concept indicating the component, and the processor outputs, as a retrieval result, the document data having the document concept that matches the first retrieval statement concept and the document concept that matches the second retrieval statement concept if at least one of the first retrieval statement concept and the second retrieval statement concept is the concept in the higher layer; or a product database that uses a hierarchical structure to store inclusive relationships among the names of a plurality of the products, wherein the document data concept extracting section further extracts the document concept indicating the product name described in the document data, on the basis of the keyword contained in said document data, the retrieval statement concept extracting section further extracts the retrieval statement concept indicating the product name described in the retrieval statement concept extracting section, on the basis of the keyword contained in the retrieval statement, the higher concept acquiring section acquires the concept that is a higher layer of the first retrieval statement concept indicating that there is the defect in the component or a state of the defect in the component, and the concept that is a higher layer of the second retrieval statement concept indicating the product name, and the generalized concept outputting section outputs, as a retrieval result, the document data having the document concept that matches the first retrieval statement concept and the document concept that matches the second retrieval statement concept if at least one of the first retrieval statement concept and the second retrieval statement concept is the concept in the higher layer.