A seat device for a vehicle comprises a lower rail fixed on the vehicle floor so as to extend in a longitudinal direction of the vehicle, an upper rail mounted to the lower rail so as to be slidable and supporting a seat body, a first lock mechanism for locking the upper rail at a first position to the lower rail and a second lock mechanism being locked for limiting the upper rail at a predetermined position so as not to move further back than the predetermined position when the upper rail is unlocked by means of the first lock mechanism, the second lock mechanism being unlocked when the upper rail is locked by means of the first lock mechanism.