1. A method of generating a lexicon group tree, comprising the steps of: (a) generating a centroid lexicon representing lexicons belonging to a predetermined lexicon group; (b) selecting two lexicons, having a longest distance therebetween in the lexicon group, using the centroid lexicon from the lexicon group, and branching a node indicating the lexicon group, based on the two selected lexicons; and (c) selecting a node having low group similarity from among current terminal nodes, including branch nodes, and repeatedly performing steps (a) and (b) on a lexicon group indicated by the selected node.