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 steady application of lubricant to rolling elements rolling through the return passage, thereby ensuring proper lubrication under maintenance-free operation. The return passage is defined inside a hole of a tubular composition installed in an axial bore made in the carriage of a slider. The tubular composition is composed of a tubular skeleton made therein with a window, and a cellular molded member that fits into the window. The rolling elements of cylindrical roller, when coming into rolling-contact with the cellular member while rolling through the return passage, are applied with lubricant film.