To realize a noble, useful seat with a simple structure designed to follow a natural movement of a seated person in front-rear and left-right directions and to support appropriately a movement of the person continuously changing its posture while the person balances a load. The seat includes the elastic support layer 3 and upper and lower base units 4 and 5 arranged to sandwich the elastic support layer 3. The upper base unit 4 includes the rolling surface 42e bulging toward the lower base unit 5 thereabove via the elastic support layer 3, is rollable in a 360° direction from the tilt reference position IRP in receiving a load of the seated person, and has a moving distal end side tilting more downward as a distance from the position IRP increases, and is configured to move while compressing the elastic support layer 3 by the rolling surface.