Measurement of stress based on the magnetoelastic interaction effect in ferromagnetic materials relies on a change in permeability due to the stress. A method for quantitatively evaluating the stress-permeability relation was proposed by combining measured magnetic flux density near a specimen with nonlinear analysis of the magnetic field by the finite-element method. First, the flux density near the specimen was measured in a low magnetic field independent of the detecting system. Next, the permeability was modeled by a linear function of the flux density. Also, the permeability was modeled to relate it with the stress in a linear form. Finally, the stress-permeability relation was determined qantitatively by comparing the measurement with the nonlinear analysis of the magnetic field based on the model of permeability.