To provide an assembling structure and an assembling method of a drive shaft enabling assembling of the drive shaft without damaging an oil seal even in case of having no differential side tube.

The oil seal 33 press-fitted to the inner circumference surface of a boss part 27c is mounted on the drive shaft 8 and a tool 40 supporting the oil seal 33 in the press-fitting direction (a) is also arranged on the drive shaft 8. An axial length b from the tip of the drive shaft 8 to the oil seal 33 is set at the length that the oil seal 33 is press-fitted to the boss part 27c with a tip part 8a of the drive shaft 8 previously spline-fitted to a side gear 28 through the inside of the boss part 27c.