To measure highly precisely the thickness of an oxide film formed on a steel plate surface at a high speed, by making a relation between emissivity and oxide film thickness on a steel plate surface which are measured with the shortest wavelength in a specified range out of a plurality of different wavelengths and with a wavelength longer than the wavelength, a relation that emissivity changes with the film thickness.

From a radiation illuminace output value of a two-wavelength radiation thermometer 20 using the shortest wavelength (2.5-10 μm) and a longer wavelength (10-20 μm) and a previously measured black body furnace output value, emissivities of the respective wavelengths are operated and obtained. Computing equipment 22 of a steel surface temperature and the oxide film thickness calculates the relation between the oxide film thickness on the steel plate 10 surface and emissivity which have been measured by the wavelengths of 2.5-10 μm and 10-20 μm, as a relation that the emissivity changes with the film thickness. The temperature of the steel plate 10 which is measured by a contact type thermometer 30 in the vicinity of the visual field of the radiation thermometer 20, measured results of the oxide film thickness which is measured by an oxide film measuring apparatus 40, and the calculated results are compared by a calculated result comparing apparatus 50, and calculation is repeated, thereby improving measurement precision.