A linear motion guide system is disclosed in which a clearance between a slider and a guide rail is closed truly to clear foreign matter away from entering inside the system through any end of the slider traveling on the guide rail. Thus, the linear motion guide system is befitting to severe working environment where much foreign matter occurs. A highly-tight sealing unit is provided which is comprised of a cassette constituted with a front panel and an enclosure, a sealing plate stowed into the cassette, and a rear panel to close an open edge of the enclosure. The sealing plate is composed of an intermediate spongy medium flanked by skin layers, and lubricant is forcibly absorbed in pores or cells in the intermediate layer.