A seat device for a vehicle includes a dynamic damper attached to a frame of a seat installed in the vehicle. The dynamic damper includes a damper case that is formed from separable first and second case halves, a weight that is housed in a damper chamber, and an elastic member that is disposed between the weight and the damper chamber so as to allow vibration of the weight. The damper case is provided with a plurality of mounting parts that are mounted on the frame, and the mounting parts comprise first and second mounting parts which are engaged with the frame at two left and right places as seen in a front view of the seat, and a third mounting part other than the first and second mounting parts which is provided between the first and second mounting parts.