1. A method of automatically generating an ontology instance, the method comprising: collecting documents corresponding to classes of an ontology; classifying the collected documents into unstructured documents and structured documents depending on whether information in the collected documents is in the form of a table; if the collected documents are unstructured documents, extracting first relationship information from the unstructured documents based on characteristics of entities, wherein the first relationship information represents relationships between entities in sentences of the unstructured document; if the collected documents are structured documents, extracting second relationship information from the structured documents based on characteristics of entities, wherein the second relationship information represents relationships between entities in a table of the structured documents; generating ontology instances from the first relationship information or the second relationship information; and mapping the generated ontology instances to corresponding classes of the ontology, wherein the extracting of the first relationship information is performed using a different process from that used to extract the second relationship information, wherein the extracting of the first relationship information from the unstructured documents includes: recognizing entity names from the unstructured documents; recognizing substitutes from the unstructured documents, wherein the substitutes are represented by pronouns in the sentences of the unstructured documents; and extracting the first relationship information from the recognized entity names and the recognized substitutes, and wherein the recognizing of the substitutes includes restoring the recognized substitutes into corresponding entity names.