A plurality of circumferential fins 22g are formed on an outer peripheral surface of a motor casing 22a, and a plurality of fins 22f are formed on an outer surface of a motor rear cover 22d. Lubricant circulates in a motor-casing oil passage 55 provided in the motor casing 22a, a motor-rear-cover oil passage 56 provided in the motor rear cover 22d, and a speed reduction portion B. This lubricant is cooled when flowing in the motor-casing oil passage 55 and the motor-rear-cover oil passage 56. Thus, an in-wheel motor drive device 21 is efficiently cooled.