Abstract: An external circulation type ball screw is provided with a helical passage in the outer surface of the nut. The helical passage and the inner helical groove of the nut are opposite in winding direction to each other. The nut is defined with two through holes which are connected to both ends of the inner helical groove and the helical passage, respectively, so that the helical passage and the inner helical groove are connected to each other to form a circulating path for the circulation of the balls. Since the helical passage winds about the outer periphery of the nut, the balls can roll more smoothly.