1. A system to recommend a keyword implemented with a computer, the system comprising: a verifying unit configured to verify whether the keyword is a direct click keyword by determining, referring to a click log, whether the keyword is associated with a first universal resource locator (URL) of an advertisement of an advertiser, and verify whether the keyword is an indirect click keyword by determining that the keyword does not locate the first URL but locates a second URL having an attribute similar to the first URL; a keyword recommending unit configured to recommend the determined indirect click keyword; and a probability computing unit configured to compute selection probabilities that the first URL is selected using the direct click keyword and the indirect click keyword, wherein the keyword recommending unit is configured to select at least one indirect click keyword having a corresponding one of the selection probabilities greater than zero from among the plurality of indirect click keywords and is configured to provide the selected at least one indirect click keyword to the advertiser, and the probability computing unit includes, a computing unit configured to compute the selection probabilities based on selection rates of the first URL and the second URL with respect to the direct click keyword and the indirect click keyword, and a similarity probability between the first URL and the second URL, and an updating unit configured to update the similarity probability based on the selection probabilities.