A gear shaft of an output gear in a speed reducer B of an in-wheel motor driving device is supported at its two ends with respect to a housing. The double-end supported gear shaft of the output gear is formed with a spline hole. The spline hole provides spline-connection with an output shaft of the wheel hub C. This structure makes the gear shaft less likely to come out of alignment even if the output gear comes under a bending moment. Therefore, even if an external force such as a lateral force from a tire causes the output shaft of the wheel hub C to develop a shaft deflection, the deformation/displacement is absorbed by a gap between splines which connect the output shaft of the wheel hub C and the output gear with each other.