A seat slides in a vehicle longitudinal direction between a pair of wheel houses projecting inside a vehicle compartment, and slides in s vehicle width direction at a portion which is in front of the wheel houses. The seat takes a first position in which its outside portion overlaps with the wheel houses in the vehicle width direction, and a second position in which its rear portion overlaps with the wheel houses in the vehicle longitudinal direction. There is provided a rearward-slide restriction device operative to restrict rearward slide of the rear seat in the first position. Accordingly, there is provided a seat device of a vehicle which can prevent any improper interference of the seat with the wheel houses, ensuring a proper long slide function of the seat.