An induction motor comprising a stator including main winding coils and sub winding coils wound in a stator core, respectively, the main winding coil having a volume greater than that of the sub winding coil, a primary rotor rotatably inserted in the stator, and a secondary rotor rotatably inserted in an air gap between the stator and the primary rotor, whereby counter electromotive force of the main winding coils is increased and resistance thereof is decreased, and thus an operation efficiency can be improved and starting voltage and break down voltage can be reduced.