shows in a graph the magnitude of transmission of vibration relative to vibration frequency between a related art example without columns and the invention with columns in addition in the motor 1 of the embodiment in addition to the increase in rigidity a device is also made to realize an increase in cooling efficiency specifically the aforesaid columns 12 a 12 b 12 c function not only to increase the rigidity of the motor case but also generate turbulent flows so as to increase heat transfer coefficient this is because although in the event that there is stagnation in a flow of cooling water heat stays or is confined in the motor case whereby the heat transfer coefficient to cooling water is reduced since turbulent flows karman vortexes are produced on the peripheries of the columns 12 a 12 b 12 c the stirring action is promoted by these turbulent flows so produced so as to reduce the stagnation of cooling water whereby the heat transfer coefficient from the stator 6 to the cooling water can be increased in the motor 1 of the embodiment since the large number of columns 12 a 12 b 12 c are disposed many turbulent flows are produced the stirring action is promoted largely so as to increase the heat transfer coefficient thereby making it possible to increase the cooling.