1. A knowledge extracting apparatus for extracting knowledge information from an electronic document, comprising: an information receiving section for receiving the electronic document; a knowledge extracting section for extracting a concept from the electronic document based on a target word to extract the knowledge information and a clue word to extract the knowledge information and forming knowledge information in which the concept thus extracted and the target word are associated with each other; a storage section for storing the knowledge information thus formed; an information analysis section for, after the knowledge information is stored, analyzing a new electronic document based on the knowledge information stored in the storage section; an electronic document searching section for searching the concept or the target word included in the new electronic document; a knowledge-registration-concept extracting section for, when at least a predetermined threshold of the concept is described in the new electronic document as a result of the searching by the searching section, the predetermined threshold of the concept being not less than 0 and not more than 1, extracting the concept, and when at least a predetermined threshold of the target word is described in the new electronic document as a result of the searching by the searching section, the predetermined threshold of the target word being not less than 0 and not more than 1, extracting the knowledge information associated with the target word from the storage section; an evaluation section for evaluating a magnitude or a fluctuation of numerical information associated with the target word or the concept extracted by the knowledge-registration-concept extracting section; and a certainty factor update section for updating a certainty factor which is an evaluation value indicative of a relationship between the concept and the target word, based on a magnitude of the numerical information, a direction of the fluctuation, and a magnitude of the fluctuation of the numerical information.