A balloon including an outer circumferential portion which dilates to form a hollow circular cross-section and deflates when the internal pressure is reduced, an inner circumferential portion positioned inside the outer circumferential portion, and support portions positioned between the outer circumferential portion and the inner circumferential portion to support deflating the outer circumferential portion while compressing the outer circumferential portion. The support portions form first compression portions, which have a high compressive strain, and second compression portions, which have a compressive strain lower than the first compression portions. When the outer circumferential portion deflates, the distance between the center of the circular cross-section and the maximum outer diameter portion becomes shorter than the radius of the circular cross-section formed when a minimum pressure, which is necessary for dilating the outer circumferential portion to have a circular cross-section, is applied to the outer circumferential portion.
