To effectively cool a motor in a housing housing type.

Intake air holes 30, 30 of cooling air of a motor 2 are formed in a rear surface of a motor housing 9, and second intake air holes 31, 31 opposed to the rear surface of the motor housing 9 are formed in a half-split housing 7 respectively. Ring-shaped ribs 32, 32 are concentrically formed on an inside surface of the half-split housing 7; a sponge ring is fitted in; the tip of a ring-shaped rib 35 formed on the rear surface of the motor housing 9 is immersed in a front face of the sponge ring 34; and an air passage 36 is formed for partitioning and connecting the intake air holes 30 and the second intake air holes 31 in a housing 5.