To provide a method and an apparatus for measuring the thickness of oxide films on the surfaces of sheet steel and capable of being applied to the outlet side of a direct-heating furnace, which is an annealing process in sheet steel manufacturing processes, and performing online, continuous, and accurate measurements on the film thickness of iron-based oxides formed on the surfaces of sheet steel.

Infrared light is intermittently irradiated to the surface of traveling sheet steel on the outlet side of the direct heating furnace. The sum energy of self-light-emitting radiant energy radiated from the surface of the sheet steel and reflected light energy of irradiated infrared light from the surface of the sheet steel is detected when infrared light is irradiated, and only the self-light-emitting radiant energy radiated from the sheet steel is detected when the irrdiation of infrared light is interrupted each in four different infrared wavelength bands. Through the use of eight detection values detected by the intermittent irradiation of infrared light, the film thickness is determined by computations.