PURPOSE: To easily alter different rim widths within a short time by reducing the set number of upper and lower rims.

CONSTITUTION: A tire 1 is held between the lower rim 3 on the side of the lower mount 11 externally fitted to a post 8 in a freely rotatable manner and the upper rim 2 on the side of a freely rising and falling advance or retreat shaft 6. The lower rim 3 is externally fitted to the lower mount 11 in a freely rising and falling state and two spacer rings 27, 28 are externally fitted to the outer periphery of the lower mount 11 between the lower end part of the lower mount 11 and the lower rim 3 in a freely revolvable and vertically movable manner. Upper protrusions 20, 31 capable of coming into contact with lower protrusions 29, 26 and capable of being inserted in the gaps between the lower protrusions 29, 26 are provided to the spacer rings 27, 28. The chuck means 40 grasping the spacer rings 27, 28 to allow them to rise and fall is provided.