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A significant overestimation of the critical current density Jc is found for a measuring method of induced third harmonic voltage for thin superconductors of thickness less than 1 µm. This is speculated to be caused by a reversible motion of flux lines in a pinning potential which reduces the nonlinear properties such as AC losses or the third harmonic voltage. It is known that the effect of reversible flux motion is prominent for superconductors with a thickness comparable to or smaller than the pinning correlation length of the order of a sub-micrometre, the so-called Campbell's AC penetration depth. In this paper the effect of reversible flux motion on the estimation of Jc by this method is preliminarily investigated by using a simplified Campbell's model for the force–displacement characteristics of flux lines.