Speech likeliness or a degree of speech is determined with a simple
configuration or with a small amount of processing, and speech parts are
separated from an input sound signal. The input sound signal is subjected
to a waveform slicing process in frame units. The increase and decrease
rate of a half wavelength in the frame is computed. The rate of a zero
cross in the frame is computed. The increase and decrease rate of a half
wavelength is computed by determining the rate of the portion where the
upward half-wavelength or the downward half-wavelength of the waveform of
the input sound signal changes to increase and decrease alternately or to
decrease and increase alternately. The degree of speech is determined
using each rate. Speech processing for separating or
accentuating/attenuating speech and background noise in accordance with
the degree of speech is performed on the sound signal for each frame.