A tire holding mechanism comprising: an upper spindle that rotatably supports an upper rim configured to abut against one opening side of a tire (5,6,7) from above the upper rim; a lower spindle that rotatably supports a lower rim configured to abut against the other opening side of the tire (5,6,7) and is configured to hold the tire (5,6,7) together with the upper rim, and has a hollow portion formed therein; a shaft that passes through the upper spindle, extends in an up-down direction of the upper spindle, protrudes further toward the lower spindle side than the upper rim, and includes a plurality of engagement grooves formed in the up-down direction at an outer periphery of a lower end of the shaft accommodated in the hollow portion; a shaft support part that has a plate part and a rotation support part which rotatably supports an upper end of the shaft; a spindle support part which includes a plate part and a rotation support part which rotatably supports the shaft, wherein the spindle support part supports the upper spindle such that, when the spindle support part moves in the up-down direction, the upper spindle also moves in the up-down direction together with the spindle support part, and wherein the spindle support part is provided below the shaft support part such that the plate part of the shaft support part faces the plate part of the spindle support part in the up-down direction, an engagement part that is provided at the lower spindle in a state of being accommodated within the hollow portion and is configured to be engaged with any one engagement groove of the plurality of engagement grooves; a first elevating part that is configured to integrally move the upper spindle, the shaft, the shaft support part, and the spindle support part in the up-down direction; and a second elevating part that is provided between an outer peripheral portion of the plate part of the shaft support part and an outer peripheral portion of the plate part of the spindle support part, wherein the second elevating part is configured to adjust a length of the shaft protruding below the upper rim by changing a distance between the shaft support part and the upper spindle in the up-down direction. an upper spindle that rotatably supports an upper rim configured to abut against one opening side of a tire (5,6,7) from above the upper rim; a lower spindle that rotatably supports a lower rim configured to abut against the other opening side of the tire (5,6,7) and is configured to hold the tire (5,6,7) together with the upper rim, and has a hollow portion formed therein; a shaft that passes through the upper spindle, extends in an up-down direction of the upper spindle, protrudes further toward the lower spindle side than the upper rim, and includes a plurality of engagement grooves formed in the up-down direction at an outer periphery of a lower end of the shaft accommodated in the hollow portion; a shaft support part that has a plate part and a rotation support part which rotatably supports an upper end of the shaft; a spindle support part which includes a plate part and a rotation support part which rotatably supports the shaft, wherein the spindle support part supports the upper spindle such that, when the spindle support part moves in the up-down direction, the upper spindle also moves in the up-down direction together with the spindle support part, and wherein the spindle support part is provided below the shaft support part such that the plate part of the shaft support part faces the plate part of the spindle support part in the up-down direction, an engagement part that is provided at the lower spindle in a state of being accommodated within the hollow portion and is configured to be engaged with any one engagement groove of the plurality of engagement grooves; a first elevating part that is configured to integrally move the upper spindle, the shaft, the shaft support part, and the spindle support part in the up-down direction; and a second elevating part that is provided between an outer peripheral portion of the plate part of the shaft support part and an outer peripheral portion of the plate part of the spindle support part, wherein the second elevating part is configured to adjust a length of the shaft protruding below the upper rim by changing a distance between the shaft support part and the upper spindle in the up-down direction.