Ferritic stainless steel is eminently suitable for use as a first wall of a fusion power reactor because of its low rate of irradiation swelling. However it is magnetizable when it is used in a magnetic field. As a result of magnetization, it will receive magnetic body forces and disturb the magnetic field configuration designed for plasma confinement. We are concerned about the range of the disturbed magnetic field and the magnitude of the induced magnetic stress and residual magnetization. Here we developed a computer program and analyzed the magnetic induction, the magnetic torque and stress for a ferromagnetic beam-plate and a ferromagnetic first wall of a fusion power reactor, considering nonlinearity of the B-H relation. We also carried out an experiment to verify the computer program. It is concluded that the extent of the disturbed magnetic field due to a ferromagnetic first wall is limited.