1. An apparatus for providing related words for a query, comprising: a communication module connected to an external client through a network to receive an input query from the client; a relational database storing data including at least one first word, at least one second word co-occurring with the at least one first word, and a co-occurrence frequency, wherein the co-occurrence frequency indicates how many times the at least one second word occurs in relation to the at least one first word; and a related word control module extracting first words included in source data by searching the source data, extracting and storing second words co-occurring by the extracted first words and the co-occurrence frequency in the relational database, wherein the related word control module is configured to search for third words co-occurring by at least one of the second words, and to determine that the at least one second word is related to at least one of the first words if the third words include the at least one of the first words, and wherein the related word control module is configured to extract the query input through the communication module and related words from the relational database, wherein at least one of the communication module, the relational database, and the related word control module comprises a processor, wherein the related word control module comprises an advanced extraction module reading the co-occurrence words by words from the relational database, extracting related words related to the word among the co-occurrence words, and storing the extracted related words in the relational database, wherein the advanced extraction module reads the co-occurrence words by specified words from the relational database, extracts N upper co-occurrence words by the specified words in the order of the co-occurrence frequency, extracts second co-occurrence words with respect to the N co-occurrence words when the co-occurrence words occur, and judges the co-occurrence word for the specified word as being a related word if the specified word among the second co-occurrence words is within the order predetermined by the co-occurrence frequency order.