A balloon including: a pressure-receiving portion formed of an elastic material and forming an enclosed space and a tubular portion formed integrally therewith. The balloon is configured to detect the air pressure in the pressure-receiving portion due to communication with a pressure detector through an opening end of the tubular portion. A transverse cross-section of the pressure-receiving portion orthogonal to the axial direction of the tubular portion has a flattened outer circumferential shape with an oblateness f=(A−B)/A, where A represents the length of a major axis of the outer circumferential shape and B represents the length of a minor axis thereof, being set to a range of 0.1≦f≦0.7 at a position at which the length A is at a maximum. The air pressure can be changed significantly and stably, and the precision of the pressure measurement can be increased.
