Seat device for vehicle

A seat device for a vehicle includes: a seat cushion and a seat base of a rear row seat, the seat base mounted on a floor of a vehicle standing upward from the floor and forming a support structure of the seat cushion; a movable cushion portion forming a sitting surface for a passenger; a seat-cushion-moving mechanism to rotate and translate the movable cushion portion so that the movable cushion portion moves between a seating position, an extended position, and a tray position. In particular, a passenger sits on the movable cushion portion in the seating position, the movable cushion portion rotates away from the seat base in the forward direction to expose a lower side of the movable cushion portion in the extended position, and the tray position is obtained when the movable cushion portion translates rearwardly from the extended position with the lower side remaining exposed.