A balloon for measuring a pressure related to the oral cavity including a pressure-receiving portion 20 that is formed of an elastic material and that internally forms an enclosed space and a tubular portion 21 that is formed integrally with the pressure-receiving portion, the balloon being configured to detect the air pressure in the interior of the pressure-receiving portion by communicating the interior of the pressure-receiving portion 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 of the major axis is at a maximum. The air pressure can be changed significantly and stably, and the precision of the measurement of a pressure related to the oral cavity can be increased.
