A balloon can maintain a bent state even if a bent state-holding member is not used. A balloon (1) includes two plastic films (10, 12) that are stacked and sealed at a peripheral edge (5). The balloon (1) includes a first part (2), a second part (3), and a bending part (4) that is situated between the first part (2) and the second part (3). The bending part (4) includes a gas passage (55) through which the interior of the first part (2) and the interior of the second part (3) communicate with each other, and a bending sealed part (54) that is formed by sealing the two plastic films (10, 12) situated opposite to each other in an inward area relative to the peripheral edge (5). An outer edge (40) of the bending sealed part (54) extends beyond the bending part (4) so as to be situated within the first part (2) and the second part (3).