7. The method of claim 1 , wherein the calculating of the relation value comprises: extracting second inquiry search words excluding the first inquiry search word from at least one of the search word history and the webpage information; calculating a similarity value between the first inquiry search word and each of the extracted second inquiry search words based on a frequency of each of the second inquiry search words being included in at least one of the search word history and the webpage information; selecting a third inquiry search word from the second inquiry search words, which is to be included in a first similar search word group together with the first inquiry search word, based on the similarity value; and revising the calculated relation value between the first inquiry search word and each of the answer candidate keywords of the first inquiry search word based on a relation value between the third inquiry search word and each of answer candidate keywords of the third inquiry search word.
7. The method of claim 1 , wherein the calculating of the relation value comprises: extracting second inquiry search words excluding the first inquiry search word from at least one of the search word history and the webpage information; calculating a similarity value between the first inquiry search word and each of the extracted second inquiry search words based on a frequency of each of the second inquiry search words being included in at least one of the search word history and the webpage information; selecting a third inquiry search word from the second inquiry search words, which is to be included in a first similar search word group together with the first inquiry search word, based on the similarity value; and revising the calculated relation value between the first inquiry search word and each of the answer candidate keywords of the first inquiry search word based on a relation value between the third inquiry search word and each of answer candidate keywords of the third inquiry search word. 8. The method of claim 7 , wherein the selecting of the third inquiry search word comprises: revising the similarity value between the first inquiry search word and each of the second inquiry search words based on a number of inquiry search words commonly existing in the first similar search word group and a second similar search word group of each of the second inquiry search words; and selecting the third inquiry search word based on the revised similarity value.