A seat apparatus includes a lower arm including one of an internal gear and an external gear, an upper arm including the other of the internal gear and the external gear, a lock member restricting and allowing a movement of a seat; a lever being rotatably connected to the lower arm, the lever being linked with an operation of the lock member to restrict the movement; and a transmission portion disposed between the lower arm and the upper arm, the transmission portion transmitting a rotation of the upper arm which corresponds to a forward inclination of a seatback to the lever by absorbing an eccentricity of the internal gear and the external gear.