A seat device for a vehicle and method of providing it

In a seatbelt device for a sub seat S, a retractor 31 to wind up a webbing 32 is attached to a sub-seat back support member 26. A buckle 42 to detachably hold a tongue 33 provided at the webbing 32 is provided on a side of the sub-seat cushion 20 which is opposite to a disposition side of the main seat M1 in a vehicle width direction. Accordingly, the seat device for a vehicle which can ensure the sufficient sitting rigidity of the sub seat and the proper support rigidity of the retractor can be provided.