An air spring comprises a substantially tapered flexible air sleeve, a piston connected to the small-diameter portion of the air sleeve, and an end cap connected to the large-diameter portion of the air sleeve. In this air spring after the inflation, the length of freely deformable portion of the air sleeve is longest at a position in its peripheral direction and shortest at a position opposite to the longest position in radial direction and is gradually changed between the longest position and the shortest position.