To provide a method for accurately measuring the thickness of a clad layer composed of a ferromagnetic material and an oxide layer formed on a nuclear fuel rod coated surface.

A probe of an eddy current sensor is arranged on the surface of the clad layer containing the ferromagnetic material. In this case, the probe is provided with a coil, the coil of the probe is energized by the AC of a first frequency passing through only the clad layer containing the ferromagnetic material and the AC of a second frequency passing through the clad layer containing the ferromagnetic material, the layer of an oxide material and a fuel rod coating and first and second complex impedances are measured. The phase of the first complex impedance is rotated, the polarity of the first complex impedance forms 180° with the second complex impedance, the second complex impedance and the first complex impedance whose phase is rotated are added, and a synthetic complex impedance for indicating the thickness of the layer composed of the oxide material is formed.