Having a stator core including a yoke, a plurality of inside teeth, and a plurality of outside teeth, an inside rotor, and an output rotor, the inside slot open angle/inside pole angle and the outside slot open angle/outside pole angle are determined so that the inside cogging torque due to rotation of the inside rotor and the outside cogging torque due to rotation of the outside rotor may cancel each other in inverted phases. Plural combinations are provided so that the amplitude of inside cogging torque and the amplitude of outside cogging torque due to rotation of outside rotor maybe approximate values in inverted phase, and the combination of the highest combined voltage value of the both induced voltage is selected.