There are provided a ball screw that more efficiently prevents accumulation and scattering of extra grease in a nut and a seal used in the ball screw. In order to do that, a grease reservoir portion (33) is provided on an inner peripheral surface of a seal (30) provided at an end of a nut (20) on rear side of a moving direction of a ball screw so as to form a space with respect to a screw shaft (10). The grease reservoir portion (33) is open to an outer peripheral surface of the seal (30) and communicates with slits (34) formed on the outer peripheral surface of the seal (30).