In a ball screw device comprising a screw shaft and a nut which make relative movement to each other through a multiplicity of balls, as well as in a linear motion device comprising an outer member and an inner member which make the relative movements to each other through the multiplicity of balls, a spacer having two concave surfaces facing respectively to balls, is disposed between the balls adjacent to each other. In one preferred form, the spacer has such a configuration that the balls adjacent to each other come into contact with at least three or more outer edge portions of the spacer or portions vicinal to the outer edges.