A linear-motion device has: a slider provided onto a guide rail relatively movably; and a plurality of rolling elements for causing the slider to move relatively in a longitudinal direction of the guide rail; whereby the slider comprising a slider block having rolling-element rolling grooves that oppose to rolling- element rolling grooves formed on the guide rail, and end caps that close both-end openings of the rolling-element rolling paths formed between the rolling-element rolling grooves on the slider block and the rolling-element rolling grooves on the guide rail, and rolling-element return paths for returning the rolling elements, which have rolled in the rolling-element rolling paths, are formed on the slider block, wherein the rolling-element return paths comprises a groove portion formed on an outer surface of the slider block along the longitudinal direction of the guide rail, and a cover member for covering the groove portion.