2. The data processing device, as claimed in claim 1 , wherein the association node joint unit has a function of categorizing the association nodes into strong association nodes which are in a strong semantic association and weak association nodes which are in a weak semantic association, and a function of joining the strong association nodes into one node.
10. The data processing device, as claimed in claim 2 , comprising: a semantic association level calculation unit for calculating a semantic association level which indicates strength of semantic association between words included in the association nodes; wherein the association node joint unit categorizes the association nodes into the strong association nodes and the weak association nodes based on the semantic association level.