A sheet identifying device includes a reader for reading a sheet in a plurality of pixels, a respective one of which includes color information having a brightness, a predetermined size of which is defined as one unit. The sheet identifying device also includes a storage section for storing image data made up of the plurality of pixels read by the reader. Further, the sheet identifying device includes an increasing/decreasing section that increases/decreases a number of pixels in the image data. In addition, the sheet identifying device includes a sheet identifying section that identifies an authentication of the sheet, based upon the image data having the number of pixels increased/decreased by the increasing/decreasing section. The sheet identifying device further includes a variable wavelength light-emitting section which is capable of irradiating a print area of the sheet with light beams having different wavelengths.