1. A speech classification apparatus that classifies speech signals into clusters based on vocal similarity, comprising: a speech classification probability calculation unit that calculates a probability that a latest input one of the speech signals sequentially input belongs to each of the clusters, based on a probability model for probabilistically determining to which cluster a certain speech signal belongs; and a parameter updating unit that successively estimates values of parameters that define the probability model using each probability calculated by the speech classification probability calculation unit and a sufficient statistic calculated based on a speech signal inputted before the latest input one of the speech signals; the speech classification probability calculation unit calculating each probability based on the probability model defined by latest values of the parameters successively estimated by the parameter updating unit.