An in-wheel motor drive assembly comprising: a wheel hub bearing unit including a tubular wheel hub, a cylindrical outer-ring-side member surrounding an outer circumferential surface of the wheel hub, and a wheel hub bearing provided in an annular space formed between the outer circumferential surface of the wheel hub and an inner circumferential surface of the outer-ring-side member and rotatably supporting the wheel hub; and a speed reduction unit adopting a speed reduction mechanism having an output shaft extending in one direction and an input shaft extending in the other direction to reduce the rotational speed of the input shaft and transmit the reduced rotation to the output shaft, a rotor unit having a tubular motor rotary shaft and a rotor secured to an outer circumferential surface of the motor rotary shaft a casing which is cylindrical and forms a contour; wherein the outer-ring-side member of the wheel hub bearing unit connects one axial end of the casing, the speed reduction unit is adapted to be inserted from an opening formed on other axial end of the casing into an interior space of the casing, wherein one end of the output shaft extending in one direction of the speed reduction unit is adapted to be fixedly inserted in the wheel hub of the wheel hub bearing unit, the rotor unit is adapted to be inserted from the opening formed on other axial end of the casing into the interior space of the casing, and wherein the other end of the input shaft extending in the other direction of the speed reduction unit is adapted to be fixedly inserted in the motor rotary shaft; and wherein the rotor unit, the speed reduction unit, and the wheel hub bearing unit are connected and assembled together. a wheel hub bearing unit including a tubular wheel hub, a cylindrical outer-ring-side member surrounding an outer circumferential surface of the wheel hub, and a wheel hub bearing provided in an annular space formed between the outer circumferential surface of the wheel hub and an inner circumferential surface of the outer-ring-side member and rotatably supporting the wheel hub; and a speed reduction unit adopting a speed reduction mechanism having an output shaft extending in one direction and an input shaft extending in the other direction to reduce the rotational speed of the input shaft and transmit the reduced rotation to the output shaft, a rotor unit having a tubular motor rotary shaft and a rotor secured to an outer circumferential surface of the motor rotary shaft a casing which is cylindrical and forms a contour; wherein the outer-ring-side member of the wheel hub bearing unit connects one axial end of the casing, the speed reduction unit is adapted to be inserted from an opening formed on other axial end of the casing into an interior space of the casing, wherein one end of the output shaft extending in one direction of the speed reduction unit is adapted to be fixedly inserted in the wheel hub of the wheel hub bearing unit, the rotor unit is adapted to be inserted from the opening formed on other axial end of the casing into the interior space of the casing, and wherein the other end of the input shaft extending in the other direction of the speed reduction unit is adapted to be fixedly inserted in the motor rotary shaft; and wherein the rotor unit, the speed reduction unit, and the wheel hub bearing unit are connected and assembled together.