Lubrication points of lubricant around a rolling element are made in a turnaround passage to ensure proper and steady lubrication by the lubrication system simple in construction, with accompanying sustainable maintenance-free condition for lubrication. The end cap has a recess as large as permitted and a hole to communicate the recess with the turnaround passage. A porous compact is composed a thick block to fit into a deep cavity in the recess, a nose leading lubricant in the turnaround passage, and a conjunctive part between the thick block and the nose. The porous compact fits snugly into the recess in a fashion that the nose comes into rolling-touch at its tip with the rollers as they roll through the turnaround passage.