An apparatus acquires learning-data, including feature-elements, to which a label is assigned. The apparatus generates a first-set of expanded feature-elements by expanding the feature-elements. With reference to a model where a confidence value is stored in association with each of a second-set of expanded feature-elements, the apparatus updates confidence values associated with expanded feature-elements common between the first- and second-sets of expanded feature-elements, based on the label. Upon occurrence of an error indicating that a score calculated from the updated confidence values is inconsistent with the label, the apparatus sets a feature-size indicating a maximum size of expanded feature-elements to be used to update the model, based on the number of occurrences of the error for the acquired learning-data, and updates the model by adding, out of expanded feature-elements generated according to the set feature-size, expanded feature-elements unmatched with the second-set of expanded feature-elements, to the model.