1. A question answering system comprising: a similar-question generation unit that selects a feature word from an input question to generate a transformational question in which the feature word is replaced with another word; a first search unit that executes a first search process based on the transformational question generated by the similar-question generation unit; a second search unit that selects a word included in a first search result acquired by executing the first search process as an extension word and executes a second search process with using the selected extension word; and an answer selection unit that analyzes a correspondence relation among extension words included in a second search result acquired by executing the second search process, determines the feature word and a constructive part of an answer equivalent word in the second search result based on an analysis result, and selects a word determined to be an answer.