PURPOSE:To prevent the generation of shaking by forming a lore rail fixed onto a chassis side and a flange part opposed in separation to an upper rail fixed onto a seat side and arranging small balls between the both flange parts and a large ball between the vertical wall parts of the rails. CONSTITUTION:The captioned adjustor 12 has a lower rail 14 fixed onto a bracket 16 fixed onto a chassis and an upper rail 15 fixed onto a seat cushion frame, and said lower rail 14 consists of a vertical wall part 20, horizontal wall part 21 extending from the both edges, of the vertical wall part 20, and a flange part 22 extending downward from the side edge of the wall part 21. Further, an upper rail 13 consists of a vertical wall part 23, horizontal wall part 24 extending from the both edges, of the vertical wall part 23, and a flange part 25 extending upwardly from the edge part of the horizontal wall part 24. Small balls 26 are arranged between the both flange parts 22 and 25, and a roller 27 having a large ball 28 which contacts the both vertical wall parts 20 and 23 inside is arranged so as to contact both horizontal parts 21 and 24.
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