To provide a bag-shaped structure that can suppress expansion of the side wall portion and improve the vascular compression property. A bag-shaped structure used in a cuff for a blood pressure monitor configured to be wrapped around a living body, inflate when a fluid is supplied to an internal space, and compress the living body includes: an inner wall portion provided on a living body side; an outer wall portion facing the inner wall portion; and a pair of side wall portions continuous with the inner wall portion and the outer wall portion and including a bent portion bent toward the internal space; and a reinforcing member provided on each of the pair of side wall portions, having a higher hardness than the side wall portion, and having shape followability in a direction of wrapping around the living body.