Possibility of the accuracy improvement in the permanent magnet method for measuring the critical current density of a high-temperature superconducting (HTS) thin film has been investigated numerically. To this end, a numerical code has been developed for analyzing the shielding current density in an HTS sample. By using the code, the permanent magnet method has been reproduced numerically. The results of computations show that, by using the magnet strength B_F as large as possible, the high accuracy can be assured with little effort. Furthermore, in order to improve the measurement accuracy of the critical current density near the film edge, it is necessary to use the magnet with the smallest radius.