A node initializing unit generates a root node including inputted phonemic models. A candidate generating unit generates candidates of a pair of child sets by partitioning a set of phonemic models included in a node having no child node into two. A candidate deleting unit deletes candidates each including only phonemic models attached with determination information indicating that at least one of the child sets has a small amount of speech data for training. A similarity calculating unit calculates a sum of similarities among the phonemic models included in the child sets. A candidate selecting unit selects one of the candidates having a largest sum. A node generating unit generates two nodes including the two child sets included in the selected candidate, respectively. A clustering unit clusters the phonemic models in units of phonemic model sets each included in a node.