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 word, at least one co-occurrence word co-occurring by words, and co-occurrence frequency; and a related word control module extracting words included in source data by searching the source data, extracting and storing the co-occurrence words co-occurring by the extracted words and the co-occurrence frequency in the relational database, and extracting the query input through the communication module and related words from the relational database.