Eddy current testing (ECT) has been widely used for the nondestructive evaluation of metallic plates or pipes. One of the methods in ECT is to measure the change of coil impedance that is caused by the metallic plate below the coil. We applied this method to estimate the thickness of an iron plate. In this case, low-frequency measurement is required in order to avoid the skin effect. At low frequencies, however, coil resistance dominates the coil impedance, and it becomes difficult to measure small changes in coil impedance. Therefore, we applied a high-temperature superconducting (HTS) coil in order to realize low coil resistance. In the experiment, changes in inductance L and resistance R of the coil were measured when the iron plate was set at 20 mm below the HTS coil. The range of the measurement frequency was chosen from 2 to 15 Hz, and the thickness of the plate was changed from 6 to 22 mm. It was shown that changes in R were clearly related to the thickness of the iron plate. Therefore, we could determine the thickness of the iron plate. Numerical simulation using the finite element method was also performed to analyze the experimental results.