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 (35, 38) 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 (34, 36, 37, 39) coupled to the intermediate shaft(s) (35, 38), the input gear, the intermediate gears (34, 36, 37, 39), 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 (34, 36, 37, 39), and at least one of the intermediate gears (34, 36, 37, 39) 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, the road wheel being adapted to be coupled to the wheel hub, whereby a space is formed between a lower part of the casing and a lower part of a rim (Wr) of the road wheel, wherein a ball joint is adapted to be coupled to a suspension member, characterized in that the ball joint is arranged in the space 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 (35, 38) 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 (34, 36, 37, 39) coupled to the intermediate shaft(s) (35, 38), the input gear, the intermediate gears (34, 36, 37, 39), 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 (34, 36, 37, 39), and at least one of the intermediate gears (34, 36, 37, 39) 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, the road wheel being adapted to be coupled to the wheel hub, whereby a space is formed between a lower part of the casing and a lower part of a rim (Wr) of the road wheel, wherein a ball joint is adapted to be coupled to a suspension member, characterized in that the ball joint is arranged in the space and the center of the ball joint is disposed immediately below the wheel hub bearing unit.