To provide a hollow body which can be expanded without using an air pump.

A side surface 14 between a bottom plate 12 and a top plate 13 is formed of a vinyl. On the internal surface 21 of a ball main body 11, a spiral skeleton 22 is extended circumferentially. Thus, this hollow body is constituted in a manner to be stretchable in the vertical direction between a contracted state wherein the top plate 13 is close to the bottom plate 12, and a stretched state wherein the top plate 13 is away from the bottom plate 12. Then, an opening 31 which communicates with the inside is provided on the top plate 13. The opening 31 is constituted in a manner closable with a cap 32.