We have proposed the noise reduction system using modified DFT pair. An input spectrum and a noise spectrum are detected at each sample using modified DFT and then noise reduction is achieved by subtracting the detected noise spectrum from the input one. We have introduced the noise subtraction coefficient in order to reduce the difference between the detected noise level and its true one. In this paper, we propose a formulation of noise subtraction coefficient for the purpose of setting the optimal coefficient automatically at each sample. The effectiveness of the proposed method is confirmed through computer simulations.