Ball screw

The present invention provides a ball screw, which may further enhance the fastening strength of the bulk with respect to the nut compared to the prior art. The ball screw 1 comprises: a screw 2 formed with a helical groove 2a on the outer circumferential surface; a nut 3 formed with a helical groove 3a on the inner circumferential surface opposite to the helical groove 2a; a plurality of balls 4 disposed by engagement with the helical grooves 2a, 3a; and a bulk 5, which forms a cyclic groove 5a on the inner circumferential side of the nut 3 for cycling of the balls 4; wherein, the nut 3 is formed with a fastening hole 3d for insertion and fastening of the bulk 5. In the ball screw 1, the outer portion of the fastening hole 3d of the nut 3 along the radial direction may become an adhesive filling portion 3f for filling the adhesive 6 to fasten the bulk 5 at the fastening hole 3d, and a side surface of the adhesive filling portion 3f is formed with a threaded groove 3g which is continued from the outer circumf