1. An apparatus for searching information based on a digital reference document containing both unstructured and structured information, comprising: a processor; and a non-transitory medium containing instructions, execution of which causes the apparatus to provide: a document converting part configured to perform a plurality of processings, respectively for generating a plurality of types of documents including a fulltext document, a section title document, an info-box document, a category document and a definition statement document, on the digital reference document, to thereby dissect the structured information of the digital reference document, each type of documents including a first field in which a unique identification of said each type of document is recorded, a second field in which a title of a page or of a hierarchical section of the digital reference document is recorded, a third field in which said each type is recorded, and a fourth field in which contents of said each type of documents are recorded; a document indexing part analyzing the fourth field of the generated documents, extracting a plurality of part-of-speech (POS)-based index terms from the fourth field of each generated document, and generating a plurality of document indices for questions and answers that are respectively a fulltext document index, a section title document index, an info-box document index, a category document index and a definition statement document index; a question analyzing part receiving a natural language question, analyzing a question pattern, an answer pattern and a question focus from the natural language question, and extracting document search keywords from the natural language question; a document searching part performing a document search, using the document search keywords, on each of the document indices for questions and answers, and generating a document search result from said each of the document indices for questions and answers; an answer extracting part extracting an intermediate answer using information about the question pattern, the answer pattern and the question focus from each of the document search result; and an answer integrating part integrating and prioritizing the intermediate answers, by assigning different weights to similar intermediate answers in accordance with specificity of each of the similar intermediate answers, wherein the more detailed similar intermediate answer receives a higher weight than a less detailed one, and correlation between a type of each natural language question and each of the plurality of types of documents, to thereby generate a final answer using both the unstructured and structured information in the digital reference document.