A ball-spline with rotary mechanism is provided which is as small as possible in size, able to perform high-precision works on a small scale and also weighs less than ever, helping the downsizing of the component-placement systems. The ball-spline has a spline shaft, a slider movable along the spline shaft through balls, and bearings installed on the opposite ends of the slider for rotation relative to a machine bed. The holder has a pair of projections which extend through slots in the spacer parts and slots in the end-cap major body to fit into slots in a carriage. Fastening screws fit into threaded holes in the projections to fasten the holder to the opposite ends of the slider.