The permeability properties of type SUS304 stainless steel (SUS304 steel) were evaluated under different values of tensile stress using the electromagnetic impedance method. The impedance–magnetic-field curve of SUS304 steel, which corresponds to the permeability–magnetic field-curve, was measured under tensile stresses of 0, 70, and 140 MPa for specimens subjected to prestrains of 5% to 40% to change the martensite fraction. The impedance curves were measured in the length (tensile) direction and the width direction. The results showed that the tensile direction was the magnetic hard axis of the martensite phase in SUS304 steel. The applied stress sensitivity of the permeability in SUS304 steel was affected by the volume fraction, residual stress, stress distribution according to the orientation angle of the martensite phase, and their interactions.