1. A computerized method for separating an original signal from a mixed signal, the computerized method comprising the steps of: a) transforming a temporal domain to a frequency domain by performing a discrete Fourier transform onto at least one of mixed signals inputted from an external device through multi-channel; b) estimating multi-decorrelation by calculating a plurality of cross power spectra for the mixed signal in the transformed frequency domain; c) estimating a separation coefficient of the mixed signal based on relative optimization in order to decorrelate the calculated cross power spectra, where the separation coefficient is serially updated; d) transforming the frequency domain to the temporal domain by performing an Inverse discrete Fourier transform on the estimated separation coefficient in the temporal domain; e) separating an original signal from the mixed signal using a processor by filtering the mixed signal using the separation coefficient of the transformed temporal domain; and wherein the step c) includes the steps of: c1) deriving an object function by applying a relative optimization for the multi decorrelation calculated at the step b) into joint diagonalization; and c2) updating a corresponding separation coefficient while calculating variance of a relative optimization convergence coefficient by performing a gradient method, a Newton's method and a trust-region method on the derived object function.