1. A noise suppressing apparatus that calculates a suppression coefficient for suppressing noise of an input signal by using a frequency spectrum of the input signal, comprising: a frequency converting section that converts the input signal into a frequency spectrum; a noise level estimating section that calculates an estimated noise level of the input signal by using the frequency spectrum; a weight coefficient calculating section that calculates N weight coefficients at predetermined intervals, N being 2 or more; and a suppression coefficient calculating section that calculates a joint distribution model of sound by weighting N statistical distribution models by using the N weight coefficients, derives an estimation expression for a sound spectrum of the input signal on the basis of posteriori probability using the calculated joint distribution model of sound as priori probability, and calculates the suppression coefficient on the basis of the derived estimation expression for the sound spectrum of the input signal and level of the input signal.