1. A question and answer database expansion apparatus comprising: a question and answer database in which questions and answers corresponding to the questions are registered in association with each other; a first speech recognition unit which carries out speech recognition for an input sound signal by using a language model based on the question and answer database, and outputs a first speech recognition result as the recognition result; a second speech recognition unit which carries out speech recognition for the input sound signal by using a language model based on a large vocabulary database, and outputs a second speech recognition result as the recognition result; and a first answer selection unit which determines whether or not a question based on the first speech recognition result and an answer corresponding to the question are registered in the question and answer database; a second answer selection unit which determines whether or not a question based on the second speech recognition result and an answer corresponding to the question are registered in the question and answer database, wherein the question detection unit calculates a first feature quantity based on the first speech recognition result, calculates a second feature quantity based on the second speech recognition result, and detects an utterance, which is not included in the question and answer database, by using the calculated first feature quantity, the calculated second feature quantity, the determination result of the first answer selection unit, and the determination result of the second answer selection unit; and a question detection unit which detects an unregistered utterance, which is not registered in the question and answer database, from the input sound based on the first speech recognition result and the second speech recognition result, and outputs the detected unregistered utterance.