A ball screw can reduce noise generation through a structure in which a ball circulates inside a nut. Besides, a return path is formed through a sleeve impregnated with a lubricant inserted in an insertion hole of a nut body, so that the lubricant is supplied to the rolling ball in the return path to achieve the aim of being free from lubricating maintenance. A nut 2 which is inserted in a screw shaft 1 with a spiral groove 11 includes a tube-shaped nut body 3 having a spiral groove 12 and a return path 13, an end cap 4, and a sleeve 6, wherein the end cap 4 is provided with an end portion guide unit 6 which is arranged at the end portion 18 of the nut body 3; the sleeve 7 is formed by a porous forming body in the insertion hole 24 of the nut body 3; and the rolling ball 9 is supplied with the lubricant when the rolling ball rolls in the return path 13 of the sleeve 7, so that the aim of of being free from lubricating maintenance can be achieved.