To improve ride comfort when inner pressure is charged and steering stability when traveling with a flat tire.

An annular support ring 4 is mounted to a wheel rim in a tire inner cavity surrounded by the tire and the wheel rim for rim-assembling the tire, and is formed of an elastic body that abuts on the inner surface of a tread section at a drop of the inner pressure of the tire to support a load. The annular support ring 4 comprises an outer ring body 5 having an outer peripheral surface 4a contactable with the inner surface of the tread section and lies on the radially outside, an inner ring body 6 having an inner peripheral surface 4b mounted to the wheel rim and lies on the radially inside, and a support wall section 7 that connects between the outer ring body 5 and the inner ring body 6 and extends circumferentially. The outer ring body 5 is provided with a plurality of through holes 10 having 3-6 mm maximum diameter and extending radially.