We have developed a new measurement device called Dual Focus Flat Cavity (DFFC) that can measure the shielding effectiveness (SE) of a material up to a frequency of 33 GHz. SE depends on the measurement setup, and therefore, characteristics inherent to materials, such as sheet resistance, should be evaluated. In this study, we estimated the sheet resistance of a material by solving the inverse problem by using the SE determined from the developed DFFC. We evaluated the estimated value by comparing it with the sheet resistance obtained using an old DFFC.