1. A classification rule generation device comprising: an input unit that inputs a document as a sample target document; a storage unit that stores extraction conditions for extracting partial text which is a portion of the sample target document and which is used for generating classification rules for classifying a classification target document to be classified into one of classification categories, the partial text being extracted from the sample target document according to the classification categories, the extraction conditions being set for each of the classification categories; a matching unit that matches the sample target document input by the input unit against the extraction conditions stored in the storage unit; an extraction unit that attempts to perform partial text extraction to extract the partial text from the sample target document according to the classification categories, based on a result of matching by the matching unit; and a learning unit that, when the partial text corresponding to one of the classification categories is extracted by the partial text extraction by the extraction unit, performs predetermined machine learning using the partial text extracted, and generates the classification rules.