An argumentation structure can be automatically extended.
A template creation unit 22 creates a template using a predetermined rewriting pattern by receiving, as an input, a proposition represented by each of nodes in an argumentation structure including the nodes respectively representing a main proposition of argumentation and a proposition that functions as an argument for another proposition. An additional proposition output unit 28 creates a proposition functioning as an argument for the input proposition using the created template and outputs the created proposition as a proposition represented by a node to be added to the argumentation structure.