1. A noise reduction device comprising: means for calculating a predetermined constant by use of an adaptive coefficient for the constant, calculating a predetermined reference signal in the frequency domain by use of an adaptive coefficient for the reference signal, and thereby obtaining estimated values respectively of stationary noise components included in a predetermined observed signal in the frequency domain and non-stationary noise components corresponding to the reference signal; means for performing a noise reduction process on the observed signal on the basis of each of the estimated values, and updating each of the adaptive coefficients on the basis of a result of the noise reduction process; and adaptive means for repeating the obtaining of the estimated values and the updating of the adaptive coefficients, and thereby learning each of the adaptive coefficients.