The rail (12) fixed to the vehicle's (10) chassis (11) has a straight first part (12b) extending horizontally in relation to the chassis on its horizontal surface (15a). The second part (12a) of the chassis is curved and attached to one end of the first part. The seat holder (13b) moves along the rail in reaction to a detected sloping position of the vehicle chassis in relation to the horizontal surface. The seat (13) fixed to the holder is constantly aligned in reaction to the vehicle chassis travelling along the horizontal and sloping surfaces. A third, curved part (12c) of the rail is attached to the second end of the first part. The second and third parts are positioned at opposite ends of the first part.
