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.