In an initial position, a movable plate (50) is rotatably connected to a fixed plate (40) by a connecting mechanism (70) (a shaft member (80) and a groove (72)). In a switching position, the movable plate (50) is slidably connected to the fixed plate (40) by the connecting mechanism (70) in a manner that enables the movable plate (50) to slide toward a seat width direction center side. When the movable plate (50) is pushed toward the seat width direction center side by the bladder (60), the movable plate (50) and a support pad are rotated from the initial position to the switching position. Then, the movable plate (50) that has rotated to the switching position is pushed toward the seat width direction center side, by the bladder (60), and the movable plate (50) and the sup port pad slide from the switching position toward the seat width direction center side.