As energy‐saving measures, the permanent magnet motor is covered a wide range of applications. Though high efficiency is realized by the permanent magnet, on the other hand the cogging torque is generated by the energy of magnet itself, and vibration and noise from the torque ripple become some problem. The development to reduce the cogging torque has been carried out from incunabula, and various techniques are developed. In this paper, based on results of FEM analysis the difference between the torque ripple by the combination of slot number and rotor pole number were examined. In addition, why it is reduced is shown analytically for the technique which adds the offset between magnet of inner R and outer R used as cogging torque reduction technique of the AC servomotor.