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Applicability of the permanent magnet method to the measurement of the critical current density in a high-temperature superconducting (HTS) thick film has been investigated numerically. A numerical code has been developed for analyzing the shielding current density in a HTS sample. By using the code, the permanent magnet method is numerically simulated. The results of computations show that the maximum repulsive force is roughly proportional to the critical current density for the case with a low critical current density. On the other hand, the proportional relation does not hold for the case with a high critical current density. In other words, there exists an upper limit of the measurable critical current density. Furthermore, it is found that the limit increases with the strength of the permanent magnet. This means that applicability of the permanent magnet method to a thick film can be considerably improved by means of a strong magnet.