gear coaxial with a motor shaft an output shaft coaxial with the motor shaft the output shaft being provided with the second internal gear and an external gear a motor core of the motor is provided with an eccentric portion eccentric to the motor shaft at a predetermined degree the external gear which has a first external gear engaged with the first internal gear and a second external gear engaged with the second internal gear is further provided with a rotor receiving portion rotatable integrally with the eccentric portion of the motor core the rotor receiving portion is coaxial with the first external gear and the second external gear application_number US 35398406 A description an embodiment of the present invention will be described hereinbelow in detail with reference to the accompanying drawings as illustrated in fig1 an entire case includes a first case 4 and a second case 3 which define a space inside thereof a motor shaft 8 is fixed to a first case 4 while an output shaft 9 is rotatably supported by the second case 3 via a bearing 11 i e a second bearing the motor shaft 8 and the output shaft 9 are arranged to be substantially coaxial with each other aside from an assembling error hereinafter when components are described.