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According to the experiments by Claassen et al., (1991), the odd harmonics of the magnetic field are excited with an increase in the amplitude of the applied ac magnetic field. Especially, the third harmonic is suddenly excited after the amplitude exceeds a certain limit. This onset of the third harmonics indicates that the critical current density is following in all over the HTS. On the basis of the experimental results, Claassen developed the contactless method for measuring the critical current density. The purpose of this study is to reproduce Claassen's results by means of the numerical simulation. To this end, the governing equation of the shielding current density has been formulated and the numerical code for solving the initial-boundary-value problem of the equation has been developed. After the shielding current density is evaluated by using the code, the magnetic field generated by the shielding current density is calculated as a function of time. Spectral analysis of the field is performed. The results of computations show that the flux-flow region covers the whole volume of the HTS above a certain limit of the amplitude of the applied magnetic field.