1. A question answering system comprising: a processor; an input device for receiving an input question sentence; a question-sentence analysis unit controlled by the processor which is configured to extract a keyword to be applied to a search process from the input question sentence; a search unit controlled by the processor which is configured to execute the search process using the extracted keyword; an answer-candidate extraction unit controlled by the processor which is configured to extract answer candidates for the input question sentence based on a search result of the search unit; a check-expression generation unit controlled by the processor which is configured to analyze a structure of the input question sentence, an analysis result of the check-expression generation unit including at least detection of a subject of the input question sentence, and to generate a check expression in which (i) at least the subject of the input question sentence of the analysis result of the check-expression generation unit and (ii) each of the answer candidates extracted by the answer-candidate extraction unit are concatenated; a check-expression search unit controlled by the processor which is configured to execute a search process based on the check expression generated by the check-expression generation unit; and an answer-candidate correction unit controlled by the processor which is configured to select an output answer to the input question from among answer the candidates based on a search result of the check-expression search unit; and an output device for displaying or storing the output answer.