1. A method of enhancing a signal employed for a signal processing application, comprising the steps of: performing spectral subtraction for obtaining an enhanced output signal by subtracting a given reference signal from a main input signal containing a target signal and a noise signal by spectral subtraction; a step of applying an adaptive filter to said reference signal; a coefficient controlling for controlling a filter coefficient of said adaptive filter in order to reduce components of the noise signal component in said main input signal, wherein said coefficient controlling comprises referencing a signal model concerning said target signal expressing a given feature concerning the target signal by means of a given statistical model, and controlling said filter coefficient is controlled based on a likelihood of said signal model with respect to said enhanced output signal, converting an acoustic signal into an electric signal using first and second signal conversion means; obtaining said main input signal by adding respective output signals from said first and second signal conversion means in a way that said target signals respectively contained in said output signals are added in the same phase; and obtaining said reference signal by adding said respective output signals from said first and second signal conversion means in a way that said target signals respectively contained in said output signals are added in the opposite phases, wherein said statistical model is based on the Hidden Markov model, and said coefficient controlling comprises updating said filter coefficient by using the EM algorithm to find a filter coefficient value which maximizes said likelihood, and replacing the value of said filter coefficient with said filter coefficient value which maximizes said likelihood, wherein said performing spectral subtraction comprises performing Fourier transformation on said main input signal and said reference signal with a predetermined frame length and a predetermined frame period, and said coefficient controlling step comprises updating said filter coefficient for every predetermined number of frames, and providing said enhanced signal with reduced noise for use in the signal processing application by a physical processing unit.