An oxide layer thickness measurement device for measuring a thickness of an oxide layer which comes into existence on a surface of a steel sheet, comprising: a layer thickness conversion information storage parts for storing a plurality of sets of layer thickness conversion information each representing a correlation between an emitting light luminance of the surface of the steel sheet and the thickness of the oxide layer; a plurality of emitting light luminance measurement parts (1-1, 1-2) for measuring emitting light luminances of the surface of the steel sheet at respective measurement wavelengths different from each other; and a layer thickness arithmetic processor for calculating, based on each of the emitting light luminances of the surface of the steel sheet measured by the emitting light luminance measurement parts, the thickness of the oxide layer by using the sets of layer thickness conversion information stored in the layer thickness conversion information storage part, wherein the sets of layer thickness conversion information respectively correspond to the measurement wavelengths, and respectively correspond to a plurality of layer thickness measurement sub-ranges constituting a predetermined layer thickness measurement range, in a specific layer thickness measurement sub-range corresponding to specific layer thickness conversion information, a ratio of a change in the emitting light luminance to a change in the layer thickness in the specific layer thickness conversion information falls within a preset extent assigned for the specific layer thickness conversion information, and the layer thickness arithmetic processor is adapted to calculate, in connection with each of the emitting light luminances of the surface of the steel sheet measured by the emitting light luminance measurement parts, the thickness of the oxide layer corresponding to the emitting light luminance of the surface of the steel sheet measured by the corresponding emitting light luminance measurement part and a ratio of a change in the emitting light luminance to a change in the thickness of the oxide layer by using the layer thickness conversion information corresponding to the measurement wavelength of the emitting light luminance measurement part, and is adapted to extract the calculated thickness of the oxide layer as a candidate value for an actual layer thickness when the calculated ratio is within the preset extent assigned for the layer thickness conversion information. a layer thickness conversion information storage parts for storing a plurality of sets of layer thickness conversion information each representing a correlation between an emitting light luminance of the surface of the steel sheet and the thickness of the oxide layer; a plurality of emitting light luminance measurement parts (1-1, 1-2) for measuring emitting light luminances of the surface of the steel sheet at respective measurement wavelengths different from each other; and a layer thickness arithmetic processor for calculating, based on each of the emitting light luminances of the surface of the steel sheet measured by the emitting light luminance measurement parts, the thickness of the oxide layer by using the sets of layer thickness conversion information stored in the layer thickness conversion information storage part, wherein the sets of layer thickness conversion information respectively correspond to the measurement wavelengths, and respectively correspond to a plurality of layer thickness measurement sub-ranges constituting a predetermined layer thickness measurement range, in a specific layer thickness measurement sub-range corresponding to specific layer thickness conversion information, a ratio of a change in the emitting light luminance to a change in the layer thickness in the specific layer thickness conversion information falls within a preset extent assigned for the specific layer thickness conversion information, and the layer thickness arithmetic processor is adapted to calculate, in connection with each of the emitting light luminances of the surface of the steel sheet measured by the emitting light luminance measurement parts, the thickness of the oxide layer corresponding to the emitting light luminance of the surface of the steel sheet measured by the corresponding emitting light luminance measurement part and a ratio of a change in the emitting light luminance to a change in the thickness of the oxide layer by using the layer thickness conversion information corresponding to the measurement wavelength of the emitting light luminance measurement part, and is adapted to extract the calculated thickness of the oxide layer as a candidate value for an actual layer thickness when the calculated ratio is within the preset extent assigned for the layer thickness conversion information.