To efficiently cool a stator and a rotor by providing an air inlet and an air outlet in a motor housing and opening air passages from the center of rotation to radial direction of the motor for the stator and rotor housed in the motor housing.

A generator-motor 2 comprises a motor housing 20 equipped with air outlets 19... (... means plurality), a cup type rotor 22, a stator 22, a housing cover 26 and a bearing 27. The rotor 22 is equipped with a predetermined number of magnets 29... on the internal peripheral surface and a plurality of air passages 33... penetrating between the inside and the outside. Also, the housing cover 26 is equipped with air inlets 40... in the central portion. Then, the rotor 22 begins to rotate together with a crank shaft 11, outside air enters into the air inlets 40... through an air inlet 12 at the side of mission 3, and this air passes through air passages 33... from air passages 53... at the side of stator 24 and moves to outside from the air outlets 19 at the side of a housing main body 21. By doing this, forced air cooling becomes possible.