In recent years, there has been an increasing demand for evaluating thick magnetic materials, such as noise suppression sheets, in the high-frequency band. However, the measurement errors caused by the demagnetizing field in thick magnetic samples have become a significant challenge. Therefore, this paper considers reducing measurement errors due to the demagnetizing field by performing 𝑆₂₁-μ conversion while considering the demagnetizing field using the finite element method, and we present the optimized permeability of Busteraid has good agreement with the conventional Nicolson-Ross-Weir method.