To provide a linear motion device decreasing stiffness of a holding piece by a simple means, and obtaining good actuating performance and durability even when a pre-load is low.

A ball circulating passage 3 is provided between an inside member and an outside member, and a plurality of balls 2 is filled in the ball circulating passage 3 so as to freely rotate. A holding piece 1b is provided between the balls 2. The holding piece 1b makes an aperture D2 of a penetrating port 13 formed on a center portion have dimensions of 35 to 65% of a diameter of the balls 2, thereby making a ratio between force between balls added on the holding piece 1b by the balls 2 and elastic displacement amount of the holding piece 1b due to the force become 0.1 to 10 [N/μm].