1. A question answering system comprising: a question-sentence analysis unit that extracts a keyword to be applied to a search process from an input question sentence; a search unit that executes the search process with using the extracted keyword; an answer-candidate extraction unit that extracts answer candidates for the input question sentence based on a search result of the search unit; a check-expression generation unit that analyzes the input question sentence to generate a check expression concatenating an analysis result of the check-expression generation unit and the answer candidates extracted by the answer-candidate extraction unit; a check-expression search unit that executes a search process based on the check expression generated by the check-expression generation unit; and an answer-candidate correction unit that selects an output answer to the input question from among answer the candidates based on a search result of the check-expression search unit.