1. A speech processing method for producing an audible speech signal based on and corresponding to an input non-audible speech signal as a non-audible speech signal obtained through an in-vivo conduction microphone, comprising the steps of: a calculating step of learning signal feature value for calculating a prescribed feature value of each of a learning input signal of non-audible speech recorded by the in-vivo conduction microphone and a learning output signal of audible whisper corresponding to the learning input signal recorded by a prescribed microphone, a learning step for performing learning calculation of a model parameter of a vocal tract feature value conversion model, which, on the basis of a calculation result of the calculating step of learning signal feature value, converts the feature value of a non-audible speech signal into the feature value of a signal of audible whisper, and then storing a learned model parameter in a prescribed storing means, a calculating step of input signal feature value for calculating the feature value of the input non-audible speech signal, a calculating step of output signal feature value for calculating a feature value of a signal of audible whisper corresponding to the input non-audible speech signal, based on a calculation result of the calculating step of input signal feature value and the vocal tract feature value conversion model, with a learned model parameter obtained through the learning step set thereto, and an output signal producing step for producing a signal of audible whisper corresponding to the input non-audible speech signal, on the basis of a calculation result of the calculating step of output signal feature value.