An invisible information detection apparatus includes parts that irradiate a light onto a medium, detect infrared light generated from the medium, and judge a specific pattern consisting of a combination of two areas on the medium in accordance with the detection result. One of the areas represents higher reflectance than the reflectance of the medium in substantially the same infrared wavelength region, and the other area represents lower reflectance than that of the medium. An invisible information detection method comprises an irradiation step, a detection step and a judgment step. The judgment step judges a specific pattern of a combination of two areas on the medium in accordance with the result from the detection step. One of the areas represents higher reflectance, and the other area represents lower reflectance, as compared to the reflectance of the medium, in substantially the same infrared wavelength region.