An in-wheel motor driving device comprising: a wheel hub bearing unit that rotatably supports a wheel hub extending in a vehicle width direction; a motor unit that drives the wheel hub; and a speed reduction unit that reduces the rotational speed of the motor unit and transmits the reduced rotational speed to the wheel hub bearing unit , wherein the speed reduction unit includes an input shaft coupled to a motor rotary shaft of the motor unit , an output shaft coupled to the wheel hub and extending in parallel with the input shaft , one or more intermediate shafts extending in parallel with the input shaft and the output shaft , an input gear coupled to the input shaft , an output gear coupled to the output shaft , and a plurality of intermediate gears coupled to the intermediate shaft(s) , the input gear , the intermediate gears , and the output gear being engaged with one another to form a driving force transmission path connecting the input gear to the output gear through the intermediate gears , and at least one of the intermediate gears has a diameter greater than the diameter of the output gear , wherein the speed reduction unit includes a casing housed in a hollow area in a road wheel (W), the road wheel (W) being adapted to be coupled to the wheel hub, whereby a space (S) is formed between a lower part of the casing and a lower part of a rim (Wr) of the road wheel (W), wherein a ball joint is adapted to be coupled to a suspension member, characterized in that the ball joint is arranged in the space (S) and the center of the ball joint is disposed immediately below the wheel hub bearing unit . a wheel hub bearing unit that rotatably supports a wheel hub extending in a vehicle width direction; a motor unit that drives the wheel hub; and a speed reduction unit that reduces the rotational speed of the motor unit and transmits the reduced rotational speed to the wheel hub bearing unit , wherein the speed reduction unit includes an input shaft coupled to a motor rotary shaft of the motor unit , an output shaft coupled to the wheel hub and extending in parallel with the input shaft , one or more intermediate shafts extending in parallel with the input shaft and the output shaft , an input gear coupled to the input shaft , an output gear coupled to the output shaft , and a plurality of intermediate gears coupled to the intermediate shaft(s) , the input gear , the intermediate gears , and the output gear being engaged with one another to form a driving force transmission path connecting the input gear to the output gear through the intermediate gears , and at least one of the intermediate gears has a diameter greater than the diameter of the output gear , wherein the speed reduction unit includes a casing housed in a hollow area in a road wheel (W), the road wheel (W) being adapted to be coupled to the wheel hub, whereby a space (S) is formed between a lower part of the casing and a lower part of a rim (Wr) of the road wheel (W), wherein a ball joint is adapted to be coupled to a suspension member, characterized in that the ball joint is arranged in the space (S) and the center of the ball joint is disposed immediately below the wheel hub bearing unit . <EOS>