To improve positioning accuracy by facilitating a shaft feed, with the deflection of a ball screw eliminated.

When a travel body 15 having a ball nut moves along a ball screw 3 rotated on the operation of a motor and the ball nut comes near an intermediate support 29, the roller 39 of the intermediate support 29 moves according to a dog 37 projected from the travel body 15 toward the ball screw 3. Consequently, the intermediate support 29 is pushed back in a direction for parting from the ball screw 3. Thereafter, the intermediate support 29 is pushed toward the ball screw 3 with an energization member 33 at a process for the ball nut to part from intermediate support 29, and a rotary support part 35 at the end of the intermediate support 29 rotatably supports the span intermediate part of the ball screw 3.