PURPOSE: To simplify the assembly of a preload device in a short time by constituting it to apply the desired preload to a ball at a time when a pair of ball nuts are rotated in a direction relatively accessible and the tip of a pin is fitted in each drill hole of the other side ball nuts.

CONSTITUTION: Drill holes 3c, 3c of a first ball nut 3 and drill holes 5a, 5a of a spacer 5, where a pin 7 is inserted through, and drill holes 4b, 4b of a second ball nut 4 are all held after shifting them in the circumferential direction, and a screw shaft 1 is fitted together with a lot of balls 2. In this state, the pin 7 of the first ball nut 3 is retracted into these drill holes 3c, 3c, and the tip is pressed to an end face, where the second ball nut 4 is opposed, by dint of the specified pressing force by a compression coil spring 6. When a pair of ball nuts 3, 4 are rotated in a direction relatively accessible, the tip of each pin 7 is fitted in each of drill holes 4b, 4b of the second ball nut 4, therefore relative motion in the circumferential direction of both these ball nuts 3 and 4 is checked. At this time, it is so designed that desired preload is set up so as to be imposed on the balls 2.