The object of the present invention is to suppress rebound of a human body by making a reaction force small due to a large damping ratio. A bulging portion 11 is formed at a seat cushioning member 14 which is a tension structure. This structure has a structure of changing the damping characteristics functioned by the change in strain energy and tension by deformation created by a plane wave of the tension structure in accordance with the magnitude of the excitation force of an inputted vibration. Therefore, since the damping characteristics function with a small damping ratio to an input of a small excitation force, it can relieve the vibration with a phase difference due to the spring property of the seat cushioning member 14 and/or a back cushioning member 24, and to an input of a large excitation force, by increase of the strain energy and decrease of the tension, the damping characteristics function with a large damping ratio and a long operating time, so that rebound of a human body upward can be suppressed.