To provide a linear motion device having excellent actuating performance, a low-noise property, and durability by suppressing falling down of a ball passage and inserting a separator sufficiently improving ball holding performance between balls, in the linear motion device provided with a ball circulating passage.

Rotations of the separator in regard to a center axis of the ball circulating passage is suppressed. An outer half portion 10OUT of the separator 10 is made to has the maximum shape not more than outer dimensions of a ball 4, and an inner half portion 10IN is made to has a shape eliminating a part contacting with an inner peripheral side guiding groove 7a of the ball circulating passage. The outer half portion 10OUT and the inner half portion 10IN are made to be asymmetrical.