An adiabatic transition field in a solenoid-induced helical wiggler has been successfully generated using staggered Ni plates with different thicknesses of 6 to 0.1 mm. However, trials using Fe plates instead of Ni plates failed to generate a suitable adiabatic transition field To elucidate these phenomena, the magnetic behavior of Ni and Fe plates with regard to the thickness dependence was studied by a fundamental electromagnetic calculation with the aid of a field calculation based on the finite element method The magnetic flux density of a Ni plate increases and then decreases with the reduction in thickness from 6 to 0.8 mm and from 0 8 to 0 1 mm, respectively On the other hand. the magnetic flux density of an Fe plate does not have a maximum but monotonically increases until the smallest thickness of 0.1 mm. The dependence of magnetic flux density on thickness differs between Ni and Fe plates This difference explains the necessity of using a Ni plate for the adiabatic transition part of the wiggler. Furthermore, we estimated three types of demagnetizing factor depending on the thickness of the plate. (C) 2009 The Japan Society of Applied Physics