1. A confidence measure calculation method of calculating a confidence measure of a target word string specified in a recognition result, the method comprising: receiving the recognition result and the target word string; generating an alternative candidate word string in a position of the target word string; training a classifier which is configured to discriminate between the target word string and the alternative candidate word string; extracting a feature value representing an adjacent context in the position of the target word string; determining whether a true word string in the position of the target word string is the target word string or the alternative candidate word string by using the classifier and the feature value; and calculating a confidence measure of the target word string on the basis of the determination result.