To provide an air cushion and a chair provided with the air cushion with which a user easily changes his/her posture and is stably supported by the balance of an air pressure after the posture is changed, easily keeps his/her position over a long period of time and is given an improved sitting feeling.

At least two or more air chambers 1 and 2 are arranged side by side in a horizontal direction and the air chambers 1 and 2 are communicated at a narrowed communication part 5. By the constitution, air moves between the air chambers 1 and 2 from the narrowed communication part 5 depending on the change of the sitting position accompanying the change of the posture and the posture is easily changed. After the posture is changed, the air pressure inside the communicated air chambers 1 and 2 is balanced and body weight is stably supported.