A sheet identifying device comprising a light-receiving section (26) for reading each pixel on a sheet which involves color information including a brightness, has a predetermined size, and is handled as one unit, a RAM (114) for storing image data constructed of read pixels, a pixel data increasing/decreasing section (116a) for increasing/decreasing the number of pixels of the image data, and a judging section for judging authentication of the sheet on the basis of the increased/decreased image data.