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This paper proposes a spectrum-shaping filter for an OFDM signal. In the proposed method, the transmitted signal after multi-carrier modulation with the inverse Fourier transform is spectrally shaped by root-roll-off filters, and the received signal before Fourier transform is also shaped by the filters. An out-of-band emission level can be significantly reduced under the condition that the transmitted data can be perfectly recovered. The paper also describes an optimized impulse response of the filter to reduce the emission level. The resultant. filter has a new impulse response different from the root-roll-off filter. The effective roll-off factor of 0.1 and the emission level of less than -100 dB are obtained with the optimization when the number of filter taps is 201 at an over-sampling rate of 4. We calculate output power spectra with the proposed filter and a conventional raised cosine window in the time domain. The calculation results show that the signal bandwidth with the proposed method can be less than one third of the bandwidth with the conventional method at an emission level of -60 dB. Therefore, the proposed method can significantly improve the spectral efficiency of OFDM systems.