1. A learning apparatus for learning shift amounts between a fundamental-frequency pattern of a reference voice and a fundamental-frequency pattern of a target speaker's voice, the fundamental-frequency pattern representing a temporal change in a fundamental frequency, the learning apparatus comprising: a computer memory capable of storing machine instructions; and a processor in communication with said computer memory, said processor configured to access the memory, the processor performing associating a fundamental-frequency pattern of a reference voice of a learning text with a fundamental-frequency pattern of a target speaker's voice of the learning text by associating peaks and troughs of the fundamental-frequency pattern of the reference voice with corresponding peaks and troughs of the fundamental-frequency pattern of the target speaker's voice; calculating shift amounts of each of points on the fundamental-frequency pattern of the target speaker's voice from a corresponding point on the fundamental-frequency pattern of the reference voice in reference to a result of the association, the shift amounts including an amount of shift in a time axis direction and an amount of shift in a frequency axis direction; and learning a decision tree by using, as an input feature vector, linguistic information obtained by parsing the learning text, and by using, as an output feature vector, the shift amounts thus calculated.