A fixed rail comprises a first rail-surface facing upward and a second rail-surface facing downward. A movable rail is configured to be slidable relative to the fixed rail and to support a seat. At least one first roller is mounted on the movable rail. The at least one first roller is configured to receive a downward load imposed on the movable rail and to roll on the first rail-surface. At least one second roller is mounted on the movable rail and is configured to roll on the second rail-surface. At least one pressing member is configured to elastically deform and to upwardly press the at least one second roller upwardly against the second rail-surface.