PURPOSE:To prevent the drop of output torque and the seizure by hollowly forming an output shaft to which a rotor is attached, and circulating cooling gas inside the hollow output shaft. CONSTITUTION:A stator 20 is attached to the inside periphery of a central housing 18 constituting a motor housing 16, and this central housing 18 is constituted in a jacket mode, and a passage 19 is provided inside, so the generated heat of the stator 20 is cooled by letting cooling fluid flow inside the passage 19. On the other hand, to the output shaft 12 wherein a hollow hole 13 is made through, a rotor 10 is fixed through a sleeve 24, and the output shaft 12 is supported through a bearing 14 before and behind the housing 16, whereby the rotation of the rotor 10 becomes possible. A fan 26 for circulating cooling gas, for example the air, inside the hollow hole 13 is attached to cool it from inside the output shaft 12, whereby the rotor 10 is cooled directly by heat conduction and at the same time the bearing is also cooled.