A search system using an extended keyword pool includes a processor. The processor includes a purchased keyword module configured to generate a purchased keyword set by searching for a keyword having a purchase history through a search advertisement. The processor includes an additional keyword module configured to generate an additional keyword set by extracting a keyword from at least one source. The processor includes a unified search keyword module configured to generate a unified search keyword set by searching for a keyword having a number of hits greater than a determined number of hits among queries comprising the purchased keyword set and the additional keyword set. The processor includes a search module configured 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.