4. The keyword ranking determining system of claim 1 , further comprising determining the main keyword among a plurality of keywords, using at least one weight selected from a weight based on a morpheme-based redundant number calculated by analyzing morphemes of the keywords included in the at least one data set, a weight according to a selection number of the document caused by the corresponding keyword, and a weight according to a length of the keyword.
4. The keyword ranking determining system of claim 1 , further comprising determining the main keyword among a plurality of keywords, using at least one weight selected from a weight based on a morpheme-based redundant number calculated by analyzing morphemes of the keywords included in the at least one data set, a weight according to a selection number of the document caused by the corresponding keyword, and a weight according to a length of the keyword. 5. The keyword ranking determining system of claim 4 , wherein the weight based on the morpheme-based redundant number is determined by whether a morpheme having a greatest redundant number is included.