To provide a linear motion device solving various problems caused by a separator and being sufficiently operated for a long period of time.

The linear motion device has a linear motion body fitted onto a shaft and linearly moved along the shaft; a large number of balls retained in ball grooves formed on an outer surface of the shaft and an inner surface side of the linear motion body and rolling in the ball grooves; and a circulation passage formed on the linear motion body and circulating/moving the ball from one end side of the ball screw to the other end side. The separator made of magnesium or a magnesium alloy which is molded by a semi-fusion molding method and in which an abrasion resistance treatment is applied onto a surface is interposed between the respective balls.