DEVICE FOR DRIVING IN-WHEEL MOTOR

Provided is an in-wheel motor drive device (21) including a motor part (A), a speed reduction part (B), a wheel bearing part (C), and a casing (22) configured to hold those parts. The motor part (A) is configured to rotationally drive an input shaft (25) of a speed reducer having eccentric portions (25a, 25b). The speed reduction part (B) is configured to reduce a speed of rotation of the input shaft (25) of the speed reducer to transmit the rotation to an output shaft (28) of the speed reducer. The wheel bearing part (C) is connected to the output shaft (28) of the speed reducer. The speed reduction part (B) includes a lubrication mechanism for the speed reduction part, which is configured to supply lubricating oil to the speed reduction part (B). Rolling bearings (37a, 37b) configured to support the input shaft (25) of the speed reducer are ball bearings each having an initial radial internal clearance (´) of from 10 µm to 60 µm.