A wheel-driving unit and a method for manufacturing the same. The circumferential-direction widths of teeth respectively constituting a male spline portion (47a) and a female spline portion (48a) are varied gradually in the respective axial directions of the male and female spline portions (47a), (48a). The directions where the widths of the teeth respectively constituting the male and female spline portions (47a), (48a) increase in the axial direction are set so as to differ from each other, whereby the spline teeth are engaged with each other in a wedge-like manner. A retaining ring (33b) is bridgingly interposed between the outer peripheral surface of the outer end portion of the spline shaft (17) and the inner peripheral surface of the near-to-outer-end portion of the spline hole (14) to thereby prevent the spline shaft (17) from slipping out of the spline hole (14).