A linear motion guide unit suitable for slender rollers, in which a tubular composition to define a return passage as well as a spacer piece to make a turnaround improve in property of retaining lubricant therein, thereby helping accomplish long-lasting service operation for lubrication. The tubular composition is composed of a tubular skeleton and a cellular member that fits into a window cut in the tubular skeleton. An end cap is comprised of an end cap major body, and two classes of spacer pieces nested in the end cap major body to define the turnaround passages. The spacer pieces of two classes are each made around outside surfaces thereof with some circular holes, which have functions of lubricant reservoirs, prevention of distortion or deformation caused in the molded spacer pieces, and ejector pinholes used to extract the molded spacer pieces out of the mold.