A linear motion guide unit is disclosed in which a tubular composition to provide a return passage is high in mechanical stiffness and also makes certain of sustainable application of lubricant to rolling elements rolling through the return passage, thereby ensuring proper lubrication under maintenance-free operation. The tubular composition to make the return passage is composed of a tubular skeleton to guide the rollers, and a porous compact impregnated with lubricant. The tubular skeleton is made up of forward and aft ends, a lengthwise column and a flexible spine, which extend between the ends. The flexible spine includes lengthwise opposite relief portions and a flexible middle section, which fit into the porous compact with separating from the porous compact to make a clearance between them. The flexible spine deflects as the rollers roll through the return passage, permitting the rollers coming into touch with the porous compact to lubricate the rollers.