1. A computer-implemented confidence measure calculation method of calculating a confidence measure of a target word string specified in a recognition result, the method comprising: a processor receiving the recognition result and the target word string; generating, with the processor, an alternative candidate word string in a position of the target word string; obtaining training data from a training data storage; training, with the processor, a classifier which is configured to discriminate between the target word string and the alternative candidate word string, using the training data; extracting a feature value representing an adjacent context in the position of the target word string; determining, with the processor, 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, with the processor, a confidence measure of the target word string on the basis of the determination result and storing the confidence measure in a calculation result storage.