A ball screw has a screw shaft including a screw groove formed in an outer peripheral surface thereof; a nut including a screw groove formed in an inner peripheral surface thereof and screwed together with the screw shaft through a large number of balls rollably disposed in a spiral-shaped ball rolling passage formed by the two screw grooves; and, two seals respectively disposed on the two end portions of the nut for sealing the opening of a clearance between the screw shaft and the nut, wherein two annular grooves are respectively formed in the inner peripheral surfaces of the two end portions of the nut, each of the seals is formed in a substantially annular-ring shape having an outside diameter larger than the inside diameter of the nut and the seals are mounted into the nut by fitting the outer edge portions of the seals into the annular grooves.