PURPOSE: To reduce the influence of variation in temperature on pitch precision, by inserting an invar core bar into the center hole of a ball screw and fixedly holding the ball screw to the invar core bar.

CONSTITUTION: A ball screw 3 on whose outer circumferential surface, the external thread 2 is formed, and into whose center hole 4, an invar core bar 5 is inserted, is made of SCM 415, for example. One end of the core bar 5 is a head 6 and the other end of it is an external thread 7. A nut 9 is threadedly engaged with the external thread 7, and the head 6 and the nut 9 are pressed onto the corresponding ends 3a, 3b of the screw 3 through washers 10, 11, respectively, for fastening the screw 3. The fastening is so performed as the ball screw assembled body 1 has its natural length at the lowest temperature of a bed which is necessary for machining or measurement. Judging through calculation from actual cross sectional area, length, coefficient of thermal expansion and Young's modulus of the screw 3 and the core bar 5, at their standard temperature, extension of the ball screw 3 caused by the variation in temperature is reduced, and the influence on pitch precision is also reduced.