A semantic network storage unit stores multiple nodes necessary for performing a task of generating a response sentence to user's speech while associating the nodes with each other. A natural language processor acquires speech information indicating content of the user's speech and identifies a primary node corresponding to the speech information from multiple nodes. A conversation generation unit selects one secondary node from multiple secondary nodes associated with the identified primary node, based on weight values each related to one of the multiple secondary nodes and generates a response sentence corresponding to the selected one secondary node.