The seatback has a seat surface curving and gradually bulging in the up-down direction, along the occupant's shoulder blades, and curving in the left-right direction and receding to support the occupant's ribs. Preferably, the seat surface of the seatback extends upward to support the lower part of the occupant's neck, thus supporting the same in the up-down direction, and recedes to support the occupant's shoulder blades in the left-right direction.