1. A speech judging apparatus comprising: an obtaining unit configured to obtain an acoustic signal including a noise signal; a dividing unit configured to divide the obtained acoustic signal into units of frames each of which corresponds to a predetermined time length; a spectrum calculating unit configured to calculate, for each of the frames, a spectrum of the acoustic signal by performing a frequency analysis on the acoustic signal; an estimating unit configured to estimate a noise spectrum indicating a spectrum of the noise signal, based on the calculated spectrum of the acoustic signal; an energy calculating unit configured to calculate, for each of the frames, an energy characteristic amount indicating a magnitude of energy of the acoustic signal relative to energy of the noise signal; an entropy calculating unit configured to calculate a normalized spectral entropy value obtained by normalizing, with the estimated noise spectrum, a spectral entropy value indicating a characteristic of a distribution of the spectrum of the acoustic signal; a generating unit configured to generate, for each of the frames, a characteristic vector indicating a characteristic of the acoustic signal, based on the energy characteristic amounts respectively calculated for a plurality of frames including a target frame and a predetermined number of frames that precede and follow the target frame, and based on the normalized spectral entropy values respectively calculated for the plurality of frames; a likelihood calculating unit configured to calculate a speech likelihood value indicating probability of any of the frames of the acoustic signal being a speech frame, based on a discriminative model that has learned in advance the characteristic vector corresponding to a speech frame as a frame of the acoustic signal including speech, and based on the generated characteristic vector; a judging unit configured to compare the speech likelihood value with a predetermined first threshold value, and judges that the target frame of the acoustic signal is a speech frame when the speech likelihood value is larger than the first threshold value: and a processor for executing computer-executable instructions associated with at least the judging unit.