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 is provided on an inner peripheral surface of a seal provided at an end of a nut on rear side of a moving direction of a ball screw so as to form a space with respect to a screw shaft. The grease reservoir portion is open to an outer peripheral surface of the seal and communicates with slits formed on the outer peripheral surface of the seal.