1. A search system using an extended keyword pool, the system comprising: a purchased keyword module to generate a purchased keyword set by searching for a keyword having a purchase history through a search advertisement; an additional keyword module to generate an additional keyword set by extracting a keyword from at least one source; a unified search keyword module to generate a unified search keyword set by searching for a keyword having a number of hits greater than a determined number, among queries comprising the purchased keyword and the additional keyword; and a search module to provide, as a keyword pool, an associated keyword or an extended keyword with respect to a search word, using the keywords in the purchased keyword set, the additional keyword set, and the unified search keyword set.