1. A retrieval apparatus, comprising: a question sentence input unit receiving a question sentence for retrieval; a retrieval execution unit retrieving data from a database and extracting data similar to the question sentence inputted by the question sentence input unit; a word contribution degree calculation unit calculating a contribution degree of a word contributing to extraction by the retrieval execution unit in a retrieval result extracted by the retrieval execution unit; and a word contribution degree output unit outputting the contribution degree calculated by the word contribution degree calculation unit together with a corresponding word.