To apply the braking force to a nut engaged with a screw shaft with a simple structure.

The nut 2 with a thread groove 2a is engaged with the screw shaft 1 with a thread groove 1a through steel balls 3 as a number of rolling elements arranged between the thread grooves 1a, 2a constituted between the screw shaft 1 and the nut 2, and an approximately cylindrical braking housing 4 for inserting the screw shaft 1, is fixed to the nut 2. A thread groove 4b corresponding to the thread groove 1a of the screw shaft 1 is formed inside the braking housing 4. A slit 4d is formed on the braking housing 4 along the longitudinal direction, an annular groove 4f is formed on an outer peripheral part 4e of the braking housing 4, and a spring ring 6 is mounted thereon. When the spring ring 6 is mounted on the annular groove 4f, the braking housing 4 is radially contracted through the slit 4d by the elastic force of the spring ring, and the thread groove 4b is pressed to the thread groove 1a. As the frictional resistance is added to the screw shaft 1 at all times, the sudden drop of the nut 2 can be prevented.