13. A reporting system tangibly embodied on a computer readable media, comprising: a document database that sequentially stores inputted 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 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 concept ratio calculating section that calculates a ratio of the number of said document data corresponding to each of said concepts to the number of said document data in said document database; a relative frequency calculating section that calculates a relative frequency indicating the magnitude of the ratio calculated by said concept ratio calculating section with respect to a reference ratio corresponding to each of said concepts; a frequent concept selecting section that selects said concepts in which said relative frequency is at least a pre-specified threshold among said plurality of concepts; a preferred concept selecting section that selects one of a first concept selected by said frequent concept selection section and a second concept corresponding to a higher layer of said first concept, on the basis of the relative frequencies of said first and second concepts; a reporting section that reports to a user that said concept of said first concept or said second concept selected by said preferred concept selecting section has a higher relative frequency; and a concept extraction rule database that stores concept extraction rules comprising sets of one or more of the keywords and the concept indicating meanings of the one or more keywords, wherein the retrieval statement concept extracting section 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 document data concept extracting section 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; 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 document 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 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 document data concept extracting section 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 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 the retrieval statement concept does not match the document concept; and a generalized concept output section that outputs 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 document data concept extracting section 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 retrieval statement concept extracting section 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 higher concept acquiring section 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 generalized concept output section 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 concept retrieving section comprises: a lower concept acquiring section that, 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, replaces the retrieval statement concept with the retrieval statement lower concept and a specialized concept output section that outputs 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 retrieval result outputting section outputs the document data retrieved by the concept retrieving section, 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 document data concept extracting section 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 retrieval statement concept extracting section 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 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 component, 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 laver; 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.
17. A computer program product comprising a computer usable storage medium having computer readable program code means embodied therein for causing functions of a reporting system, the computer readable program code means in said computer program product comprising computer readable program code means for causing a computer to effect the functions of claim 13 .