A storage unit stores at least one attribute each assigned an attribute identifier uniquely identifying the attribute. Each attribute identifier is associated with at least one piece of equivalent reference data each representing the attribute. Each attribute identifier is not a word representing the attribute, and is made of a symbol having no meaning per se. A weight of association between an attribute identifier and each of at least one piece of equivalent reference data represents a degree of relatedness between the attribute and the piece of equivalent reference data. An operation unit determines whether or not given data is the same as a piece of equivalent reference data, obtains a weight of association between the piece of equivalent reference data determined to be the same and an attribute identifier, and selects the attribute when a sum of at least one weight obtained for the attribute satisfies a predetermined condition.