1. A keyword ranking determining system, comprising: a computer readable non-transitory storage medium; a data grouping unit stored on the storage medium and configured to group data of a weblog according to a theme; a weight application unit configured to calculate a document concentration that denotes a concentration degree, with respect to the theme, of a document corresponding to the data grouped according to the theme and to apply a weight corresponding to the document concentration to the data; a ranking determination unit configured to generate at least one data set by grouping the data applied with the weight according to a search intention and to determine rankings of the at least one data set according to the theme; and a main keyword determination unit configured to determine a main keyword representing each of the at least one data set.