An object of the present invention is to provide an air cell which is capable of detecting that the upper end side and the lower end side of the air bag are nearer than a predetermined distance. This air cell has an air bag, a base member to which the lower end portion of the air bag is attached, an air chamber AR configured by the air bag and the upper surface of the base member, a thin film portion which forms the bottom surface of the air chamber and capable of deforming elastically in the up-and-down direction, an extending member configured to extend upwardly from the upper surface of the thin film portion, detecting means which is provided under the thin film portion and which detects downward deformation of the thin film portion.