A seat belt device for a vehicle includes a buckle head mounted on a seat and designed to couple with a tongue and a carriage unit allowing the buckle head to be displaced in horizontal and vertical directions of the seat so that the buckle head and the tongue are easily coupled to or decoupled from each other when a belt is fastened or unfastened. The seat belt device facilitates fastening of a seat belt more conveniently by allowing for the horizontal and vertical displacement of a buckle head.