SEAT SLIDE ADJUSTER

Rattling and abnormal sound between an upper rail and a lower rail are reduced, and sliding is more smoothly performed. Sliding resistance reducing members (14, 15) include retainers (141, 151) formed each in a substantially L shape in a cross section and including first retaining wall portions (1411, 1511) and second retaining wall portions (1412, 1512) located between vertical wall portions (11b, 12d) and between bottom wall portions (11a, 12c) of a lower rail (11) and an upper rail (12). The first ball members (142, 152) are disposed in middle positions of the first retaining wall portions (1411, 1511), and the second ball members (143, 153) are disposed in positions apart from the vertical wall portions toward the center in the width direction of the lower rail (11) in the second retaining wall portions (1412, 1512).