A seat device for a vehicle comprises a seat for a passenger including seatbacks to support a passenger's back portion, reinforcing structure bodies provided inside the seatbacks along an outer periphery of the seatbacks, headrests provided at upper portions of the reinforcing structure bodies to support a passenger's head portion, connection support portions provided beside the reinforcing structure bodies to connect the reinforcing structure bodies to a vehicle-body member, and move-allowance portions to allow lower portions of the reinforcing structure bodies to move rearwardly in a vehicle rear collision so that the headrests move forwardly or upwardly. Accordingly, a passenger's head portion can be protected effectively with a simple structure.