In the ball screw, a ball circulation tube 20 is formed to have such a shape that the movable amount of balls 15 in a first direction (X) intersecting at right angles to the axis of the tube 20 is larger than the movable amount of the balls 15 in a second direction (Y) intersecting at right angles to the first direction (X), and the ball circulation tube 20 is fixed to a nut 13 in such a manner that the outer surface 20a of the tube 20 extending in parallel with the first direction (X) is opposed to the outer surface of the nut 13. Accordingly, it is possible to provide a ball screw which can restrict variations in the torque of the ball screw caused by the mutual sliding movements of balls occurring within a ball circulation tube.