To easily prevent an accessary part from being damaged due to assembly in a method for assembling a tire on a rim outer surface of a wheel to which the accessary part is mounted beforehand.

The wheel is positioned in an initial position by partially fitting the tire 16 on the wheel 12 obliquely from an upper part. Extending tools 30 and 32 for extending a pair of beads 72 and 74 of the tire along the radial direction of the wheel are turned around a rotational axis of the wheel from the state positioned in the initial position. In the initial position, the tire is positioned with respect to the wheel in a state that the accessary part 92 and a lower side part 100 positioned on a lower side than an upper rim flange 82 positioned on an upper side of a pair of the rim flanges 82 and 84 of the wheel and fitted in the wheel of the lower bead 74 positioned on a lower side of the pair of the beads are not overlapped.