This paper presents a topology optimization method for a permanent magnet synchronous motor (PMSM), considering the nonlinear ferromagnetic material property. Based on the density approach, we propose an interpolation model using the relationship between the magnetic field intensity and the magnetic flux density. Since the appropriate value of the penalty parameter of the density approach is unclear when dealing with the nonlinear ferromagnetic material, we conducted a numerical experiment to determin the valid range of the penalty parameter. We clarified that the optimized configurations are independent of the penalty parameter settings and free from gray-scales. Additional numerical experiments are conducted to investigate the usefulness of the proposed method by solving a torque maximization problem for the design of PMSM.