A boundary estimation method readily learns the probability of existence of a boundary in speech. In a learning mode, estimators 6002, 6001 estimate distributions of boundary samples and non-boundary samples. In an estimation mode, a likelihood calculator 9001 calculates a likelihood of a boundary from the outputs of a boundary probability density calculator 8002 and a non-boundary probability density calculator 8001. A feature extractor 1 analyzes the input speech to convert it into feature parameters of time series, a boundary detector detects phonetic boundary equivalent areas in the input speech from the output of the feature extractor, a model calculator prepares a plurality of phonetic model series corresponding to the feature parameters and restricts a time when the boundaries of the phonetic model series are formed to the phonetic boundary equivalent areas detected by the boundary detector, and a phonetic series transform selects suitable phonetic model series corresponding to the input speech from the result of the model calculator. <IMAGE>
Boundary estimation method