in a seat sliding device comprising a stationary rail , a movable rail slidable on and along the stationary rail to carry a seat , and a plastic slide block moulded on the movable rail in a manner to be disposed between these two rails to improve the sliding characteristics of the movable rail relative to the stationary rail , there is proposed a measure in which the slide block is divided into two sections in the longitudinal direction of the movable rail , leaving a predetermined clearance between the two sections .