Abstract: An automatic assembling method and apparatus for a differential gear, for simplifying the structure of the apparatus and achieving positive assembling without damage to gear teeth of gears, wherein while positioning a pair of side gears R, R at predetermined positions in a differential case C, a pinion gear P and a spherical washer Q are fitted to coincide with each other at shaft insertion bores thereof so as to be inserted with an internal chuck 14 from inside of the pinion gear P (i.e., center side of the differential case), thereby the pinion gear P and the spherical washer Q are held together at respective internal surfaces. Thereafter, the pinion gear P is meshed with the side gear R at gear teeth portions thereof at an opening H of the differential case C, and the pinion gear P is moved to an assembling position while holding the side gear rotatable. A shaft S is then inserted from a shaft hole e of the differential case C, thereby assembling the differential gear.