A motor/generator is basically provided with a stator, a first rotor member and a magnetic resistance changing mechanism. The stator has a plurality of coils configured and arranged to be energized with a composite electrical current to form first and second magnetic fields. The first rotor member is configured and arranged to be rotated with respect to the stator using the first magnetic field. The magnetic resistance changing mechanism is configured and arranged to change a magnetic resistance between the stator and the first rotor member using the second magnetic field.