We propose a modified version of the Fourier self-deconvolution (FSD) filter to improve the resolution of spectrum bands intrincically broadened. The fundamental concept is similar to the FSD method developed by Kauppinen et al. [Appl. Spectrosc. 35, 271 (1981)] and to the Finite Impulse Response Operator (FIRO) method formulated by Jones and Shimokoshi [Appl. Spectrosc. 37, 59 (1983)]. Our method, however, requires less computation time than the FSD method and gives shorter filterlength than the FIRO. The filter finally obtained is equivalent to the FIRO multiplied by the appropriate window function. To demonstrate the effectiveness of the method, it is applied to simulated spectra and an infrared spectrum of chlorobenzene.