A seat belt assembly for a vehicle may be arranged to facilitate installation of a seat belt while improving the operation and feel during use. The vehicle may have a pair of left and right occupant seats on a vehicle body frame. A pair of left and right standing frames may be disposed on the outside of the seats, and the upper end parts of the pair of standing frames may be connected by a cross frame. A winding device of the belt may be supported on the frame so as to be positioned in back of the seat. A through-anchor into which the belt may be inserted may be supported on the upper part of the standing frame. Guide ribs may be provided on a back surface of the seat between the winding device and the through-anchor so as to guide the movement of the belt.