1. A pitch estimating method comprising: computing a normalized autocorrelation function of a windowed signal obtained by multiplying a frame of a speech signal by a window signal, and determining candidate pitches from a peak value of the normalized autocorrelation function of the windowed signal; interpolating a period of the determined candidate pitches and an estimated candidate pitch value within the interpolated candidate pitch period; generating Gaussian distributions for the candidate pitches for each frame for which the interpolated estimated candidate pitch value is greater than a first threshold value; mixing the Gaussian distributions which are located at a distance less than a second threshold value to generate mixture Gaussian distributions and selecting at least one of the mixture Gaussian distributions that has a likelihood exceeding a third threshold value; and executing dynamic programming for the frames based on the candidate pitches of each of the frames and the selected mixture Gaussian distributions to estimate the pitch of each frame.