PURPOSE: To efficiently and properly assemble a rim without falling-off of a bead part by disposing a base-form bead part falling preventing means having an extended part larger than the diameter of a wheel rim flange, in the vicinity of the lower part of a lower side rim flange.

CONSTITUTION: A working base 6 erected on the center of a base 2 is provided with a fixed shaft 3, the boss hole 14 of a wheel 10 is fitted to a wheel holding shaft on the upper end of this fixed shaft 3 so as not to rotate and the boss part 13 of the wheel 10 is supported from under by a supporting part 5 provided on the fixed shaft 3. On the working base 6 thus structured, a base-form bead part falling preventing means 7 having an extended part 18 larger than the diameter of a rim flange is disposed in the proximity of the lower part of the end of a lower side rim flange 15. Thus, on the extended part 18 the lower side bead part 51 of a tire 50 is supported from under and falling of the lower side bead part 51 from a rim part 11 during fitting of an upper side bead part 52 into the rim part 11 is prevented.