A slider assembly configured to move on a slide rail along an axial direction without limitation in distance is provided to solve the problems of the middle retaining members of a conventional circulation maintaining device, e.g., insufficient connection rigidity and inconvenience in manufacture and assembly. The positioning portion of a middle retaining member and the positioning portion of the protrusion of a slider are connected in a direction perpendicular to both the axial direction and a transverse direction in which the protrusion projects, thereby connecting the middle retaining member and the protrusion securely and evenly along their entire lengths. The middle retaining member and the protrusion also feature ease of manufacture and assembly.