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We fabricated arrays of subwavelength slits in an Al film and measured their optical properties in the visible light region. Al crossbeams were installed as supports between the Al lines to enable us to precisely fabricate the nanoslits. Visible light polarized parallel to the slits transmits at shorter wavelengths, similar to that of a wire grid array at infrared wavelengths. On the other hand, for light polarized perpendicular to the slits, the transmission is at longer wavelengths and significant absorption takes place at shorter wavelengths due to Fano-type interference. This absorption in the visible wavelength region is due to the precise nanofabrication process and the high plasma frequency of Al. These results show that Al nanoslits can be used as polarization filters in the visible wavelength region.