This air-cooled type in-wheel motor drive assembly 21 includes a lubrication oil circuit for lubricating a speed reduction unit B by allowing lubrication oil discharged from a lubrication oil pump 51 to flow through interconnected oil passages 54, 55, 56 provided to a casing 22, motor-rotary-shaft oil passage 57, speed-reduction-unit input-shaft oil passage 58 and interior of the speed reduction unit B, and for cooling a motor unit A and the speed reduction unit B with the lubrication oil cooled with the aid of outer fins 22f while flowing through the oil passages 54, 55, 56 provided to the casing.