There is provided a seat device for a vehicle comprising a pair of main seats disposed in the vehicle width direction including respectively a main seat cushion and main seat back, and a sub seat back disposed between the main seat backs, wherein the sub seat back is configured so as to be in an arranged position in which the sub seat back is located above the main seat cushion and before the main seat back, and the main seat backs are configured so as to be moved inward when the sub seat back is in the arranged position. Accordingly, the width of the seat can be adjusted variably, without improperly affecting spaces behind or before the seat, thereby attaining both the movement of seat and the improvement of space utility.