Abstract: This ball spline consists of a spline shaft provided with first raceway grooves, and a cylindrical casing mounted on the spline shaft so that the cylindrical casing can be moved slidingly on and relatively to the spline shaft. The cylindrical casing has a cylindrical casing body, end caps fixed to both ends of the cylindrical casing body, end seals attached to end surfaces of the end caps, and balls rolling between the spline shaft and cylindrical casing body. The cylindrical casing body consisting of a cylindrical casing member provided with return passages and formed out of a metal or synthetic resin material, and a liner member fitted in the cylindrical casing member and provided with second raceway grooves in which the balls roll.