A liner motion device has a liner motion element fitted on a shaft and moving straight along the shaft; a plurality of balls which are retained in a rolling element groove formed on the inner side of the liner motion element and roll over between the rolling element groove and the shaft; a separator interposed between the rolling elements; and, a circulating path formed in the liner motion element through which the rolling elements are circulated from one end of the rolling element groove to the other, wherein the separator is a product of a synthetic resin containing a lubricating oil in an amount of 3 to 25% by volume.