The present invention provides an FFT window spread generating method used in the modulation of an OFDM system. When OFDM-modulated received signals each having a time length greater than or equal to an effective symbol length are demodulated by FFT processing using a spread FFT window, the spread FFT window makes use of received signals each having the time length greater than or equal to the effective symbol length. Spreading windows spread before and/or after the effective symbol length are provided. Received signals lying within the spreading windows are added to their corresponding received signals lying within the effective symbol length and different in time position by the effective symbol length, and the amplitudes of the added received signals are reduced to half respectively, thereby generating the spread FFT window.