Demagnetization is crucial in national defense and is applied by deperming protocols. However, because a warship has an open structure, it is inevitable that demagnetizing fields are generated and magnetic fields that are
different from the intended are applied. Therefore, in this paper, the effect of the demagnetizing field during the demagnetization process was analyzed. The magnetization distribution was simulated using a program combining the finite element method and Preisach model, and verified through experiments using specimens. In addition, experiments using scaled-down warship were performed. From the experimental results, we confirmed that Deperm-ME is superior to Anhysteretic, because the internal applied magnetic field was constantly applied.