PURPOSE: To improve and simplify a troublesome assembling process in which case-side friction plates and shaft-side friction plates are alternatively assembled one by one in a narrow space inside a differential case.

CONSTITUTION: A parking braking device has a plurality of braking discs 17 on the side of an differential shaft 12 and a plurality of friction plates 18 fitted to an inner peripheral surface of a differential case 10 in an assembly arrangement, and an operation mechanism B which can freely make the friction plates 18 and the braking discs 17 be in press-contact and release it. A boss member 16a is slidably spline-fitted to the differential shaft 12 from outside, while the braking discs 17 are spline-fitted to the boss member 16 from outside with escape prevention on both sides in an axial direction. Namely, four braking discs 17 and three friction plates 18 are preliminarily assembled on the boss member 16 for structuring a boss assembly.