PURPOSE: To make it possible to use a cuff whose exhaust ability is suddenly decreased at a specified pressure value as a border in a pressure range of blood pressure measurement by promoting elastic deformation under a low pressure of a gas ventilation hole formed by a gap at an opening part by means of a repulsion force of torsion of a thin sheet part.

CONSTITUTION: When a vacuum, for example, of 250-150mmHg is applied to a flat part 1b of a main body 1, an orifice with a triangle shape determined by the height of an apex parts 2b of a thin sheet member 2 pinched at an opening part 1d is constituted and it becomes a part of a straight line of a characteristic curve of a change in area of the orifice opening part. When a pressure of 150-60mmHg is applied, a semiellipse-shaped orifice whose one side is constituted of the part of the height of the apex part 2b is constituted by an elastic recovery of the main body 1 and it becomes a part of a straight line of the characteristic curve A. When a pressure of 60-0mmHg is furthermore applied, at the apex part 2b, torsional recovery force of the thin sheet member 2 becomes larger than the elastic recovery force and it results a sudden change in area of the orifice opening part. It is thereby possible to perform a const. cuff expiratory actioity even when the cuff expiratory ability is suddenly decreased.