A sound quality improvement method for a noisy speech signal according to an embodiment of the present invention comprises the steps of estimating a noise signal of an input noisy speech signal by performing a predetermined noise estimation procedure for the noisy speech signal; measuring a relative magnitude difference to represent a relative difference between the noisy speech signal and the estimated noise signal; calculating a modified overweighting gain function with a non-linear structure in which a relatively high gain is allocated to a low-frequency band than a high-frequency band by using the relative magnitude difference; and obtaining an enhanced speech signal by multiplying the noisy speech signal and a time-varying gain function obtained by using the overweighting gain function. Accordingly, the amount of calculation for noise estimation is small, and large-capacity memory is not required. Furthermore, the present invention can be easily implemented in hardware or software, and the accuracy of noise estimation can be increase because an adaptive procedure can be performed on each frequency sub-band.