1. A search system using an extended keyword pool, the system comprising: a processor including 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 including an additional keyword module configured to generate an additional keyword set by extracting a keyword from at least one source; the processor including 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 during a determined period; and the processor including a search module configured to, provide an associated keyword or an extended keyword with respect to a search word, using an extended keyword pool, the extended keyword pool being generated such that the purchased keyword set, the additional keyword set, and the unified search keyword set are connected to one another, provide the associated keyword or the extended keyword with respect to the search word based on new keyword scores or associated scores of keywords included in the extended keyword pool, increase the new keyword scores of the keywords included in the unified search keyword set, excluding the keywords included in the purchased keyword set and the additional keyword set, and increase the associated scores of intersection keywords commonly included in the purchased keyword set and the additional keyword set.