In order to improve steering operation operability during cornering while maintaining a driver holding property, a seat apparatus is provided with: a seat back frame; a cornering sensor for detecting a cornering direction of a vehicle; and a control unit that controls the seat back frame on the basis of the result of detection by the cornering sensor. The seat back frame is provided with: left and right side frames disposed on both sides of the seat back frame; a back plate disposed between the left and right side frames; and a swinging mechanism for swinging the back plate with respect to the direction of movement of the vehicle. The control unit controls the swinging mechanism such that, upon detection of a cornering direction of the vehicle by the cornering sensor, the inner one of the sides of the back plate with respect to the cornering direction is positioned rearward of the outer one of the sides with respect to the cornering direction.