1. A warping factor estimation apparatus comprising: a label information generation unit configured to generate information on voice and non-voice labels, the labels being obtained based on an acoustic feature and a corresponding written text; a warp model storage unit configured to store a probability model representing voice and non-voice occurrence probabilities, parameters of the probabilities being obtained using the acoustic feature; and a warp estimation unit configured to receive a cepstrum extracted from digital input speech data, the digital input speech data being obtained on analog-to-digital conversion of an input speech signal, to read the probability model representing voice and non-voice occurrence probabilities from the warp model storage unit, and to receive the voice and non-voice labels from the label information generation unit, the warp estimation unit configured to calculate a warping factor in a frequency axis direction using the probability model representing voice and non-voice occurrence probabilities, the voice and non-voice labels, and the cepstrum to output a resulting warping factor.