A linear motion guide unit has a guide rail made of a thin plate, and a stopper construction installed detachably to the guide rail to prevent a slider from becoming untethered while on a sliding movement relative to the guide rail. A linear motion guide unit is disclosed in which a slider fits for linearly sliding movement in a guide rail made of a thin steel material formed in an U-shape in cross section, and there is provided a stopper construction to keep the slider against getting out of the guide rail. The stopper construction is comprised of a stopper pin installed detachably to the guide rail at any lengthwise end thereof to cause the stopper pin to bear against any fore-and-aft end of widthwise opposing side walls of the slider, whereby the slider comes to rest to prevent it from becoming untethered from the guide rail.