A linear motion guide unit has a stopper plug that is first set on any one of the forward and aft ends of the guide rail in place and then fastened to the guide rail against turning around its own lengthwise direction by using just one screw. The stopper plug has horn-like projections at areas corresponding to the raceway grooves on the guide rail, the horn-like projections spreading sidewise beyond contour of the guide rail. The horn-like projections are made with raised portions. Upon movement of the slider relatively to the guide rail, the retainer collides at forward and aft ends thereof against the raised portions of the stopper plugs, which fit in the raceway grooves on the guide rail, to keep the retainer against escape out of the guide rail.