The cooling air introducing port 24 is open to a lower portion of an axial direction end portion of the motor housing member 11, 12, the water-proof cover 25 for covering the cooling air introducing port 24 is attached to the outside of the axial direction end portion of the housing member 11, 12, and the air introducing passage 26 having a labyrinth structure is composed of the bent portion 25d provided in a lower end portion of the water-proof cover 25, the first wall face 21d provided in the annular portion 21b of the shroud and the second wall face 21e.