An in-wheel motor drive device, comprising: a motor portion having a cylindrical motor casing, a motor rear cover that covers an opening located on one side in an axial direction of said motor casing, a plurality of cooling fins that are formed on at least one of an outer peripheral surface of said motor casing and an outer surface of said motor rear cover, and a motor rotation shaft that is provided in said motor casing to output rotation to the other side in said axial direction of said motor casing; a speed reduction portion which has a cylindrical speed-reduction-portion casing having its one side in said axial direction connected to the other side in said axial direction of said motor casing, an input shaft provided in said speed-reduction-portion casing and coupled to said motor rotation shaft, and an output shaft, and which reduces a speed of rotation of said input shaft to transmit said rotation having the reduced speed to said output shaft; a wheel hub that is rotatably supported in an opening located on the other side of said speed-reduction-portion casing, and is fixed and coupled to said output shaft; and a lubricant circulation passage which has an oil passage (58a, 58b) provided in said motor casing having said cooling fins formed thereon or in said motor rear cover having said cooling fins formed thereon, and which allows lubricant to circulate in said oil passage (58a, 58b) and said speed reduction portion to lubricate inside of said speed reduction portion, wherein said oil passage (58a, 58b) includes a motor-casing oil passage provided in said motor casing, and said cooling fins are formed in a region of said outer peripheral surface of said motor casing where said motor-casing oil passage is provided, and wherein a motor-casing ridge is formed on said outer peripheral surface of said motor casing, said motor-casing oil passage is provided inside said motor-casing ridge, and said cooling fins extend from said motor-casing ridge. a motor portion having a cylindrical motor casing, a motor rear cover that covers an opening located on one side in an axial direction of said motor casing, a plurality of cooling fins that are formed on at least one of an outer peripheral surface of said motor casing and an outer surface of said motor rear cover, and a motor rotation shaft that is provided in said motor casing to output rotation to the other side in said axial direction of said motor casing; a speed reduction portion which has a cylindrical speed-reduction-portion casing having its one side in said axial direction connected to the other side in said axial direction of said motor casing, an input shaft provided in said speed-reduction-portion casing and coupled to said motor rotation shaft, and an output shaft, and which reduces a speed of rotation of said input shaft to transmit said rotation having the reduced speed to said output shaft; a wheel hub that is rotatably supported in an opening located on the other side of said speed-reduction-portion casing, and is fixed and coupled to said output shaft; and a lubricant circulation passage which has an oil passage (58a, 58b) provided in said motor casing having said cooling fins formed thereon or in said motor rear cover having said cooling fins formed thereon, and which allows lubricant to circulate in said oil passage (58a, 58b) and said speed reduction portion to lubricate inside of said speed reduction portion, said oil passage (58a, 58b) includes a motor-casing oil passage provided in said motor casing, and said cooling fins are formed in a region of said outer peripheral surface of said motor casing where said motor-casing oil passage is provided, and wherein a motor-casing ridge is formed on said outer peripheral surface of said motor casing, said motor-casing oil passage is provided inside said motor-casing ridge, and said cooling fins extend from said motor-casing ridge.