Magneto–impedance appears at remarkably high levels in amorphous alloys, and is used in high–sensitivity magneto–sensors made from these materials. It can also be used in non–destructive sensing systems, because it is strongly dependent on magnetic anisotropy. We therefore developed an electromagnetic ac impedance wave method (EMIW) that measures the magneto–impedance of ferromagnetism without contact. The effect of applied stress on the impedance waves of steel and nickel, under plastic deformation, was measured by EMIW, and the impedance waves were measured while decreasing the applied stress after plastic deformation. During the plastic deformation process, the applied stress effect of SM490A could be measured, but in nickel samples the changes in the impedance waves declined with decreasing stress. It was also found that the effect of applied stress on the impedance waves was different in loading and unloading.