1. A question answering system comprising: a question answering unit that executes search processing on a basis of an input question and acquires answer candidates to the question; an analysis unit that executes syntactic analysis processing or syntactic and semantic analysis processing on a passage obtained as a result of the search processing executed by the question answering unit; a tree structure generation unit that generates a tree structure on a basis of an analysis result of the analysis unit; a feature extraction unit that extracts a relation between a search word applied in the search processing executed by the question answering unit and each acquired answer candidate as a feature corresponding to each answer candidate from the tree structure generated by the tree structure generation unit; an evaluation unit that determines an evaluation value of each answer candidate on a basis of the feature extracted by the feature extraction unit; and a re-ranking unit that re-ranks the answer candidates on a basis of the evaluation values determined by the evaluation unit.