1. A clustering apparatus comprising: an input unit configured to input at least one phonemic model attached with determination information indicating a small amount of speech data for training, and at least one phonemic model not attached with the determination information; a node initializing unit configured to generate a node including the inputted phonemic models as a root node of a tree structure; a candidate generating unit configured to generate candidates of a pair of child sets for a node having no child node among nodes in the tree structure, by partitioning a set of phonemic models included in the node into two; a candidate deleting unit configured to delete a candidate including the two child sets at least one of which includes only phonemic models attached with the determination information, from the generated candidates; a similarity calculating unit configured to calculate a similarity among the phonemic models included in each of the two child sets included in each of the candidates other than the deleted candidates, and calculates a sum of the similarities calculated for the child sets; a candidate selecting unit configured to select one of the candidates having a largest calculated sum; a node generating unit configured to generate two nodes including two child sets included in the selected candidate, respectively, as child nodes of the node that is a generation source of the selected candidate; and a clustering unit configured to cluster the phonemic models in units of phonemic model sets included in nodes of the tree structure.