[Problems] To convert a signal of non-audible murmur obtained through an in-vivo conduction microphone into a signal of a speech that is recognizable for (hardly misrecognized by) a receiving person with maximum accuracy. [Means for Solving Problems] A speech processing method comprising: a learning step (S7) for conducting a learning calculation of a model parameter of a vocal tract feature value conversion model indicating conversion characteristic of acoustic feature value of vocal tract, on the basis of a learning input signal of non-audible murmur recorded by an in-vivo conduction microphone and a learning output signal of audible whisper corresponding to the learning input signal recorded by a prescribed microphone, and then, storing a learned model parameter in a prescribed storing means; and a speech conversion step (S9) for converting a non-audible speech signal obtained through an in-vivo conduction microphone into a signal of audible whisper, based on a vocal tract feature value conversion model, with a learned model parameter obtained through the learning step set thereto.