<p>A ball screw device in which balls (3) can be smoothly circulated in an endless circulation passage by improving the stability of the behavior of the balls (3) moving from a loaded state into a non-loaded state or vice versa at the end of a loaded rolling groove (21) in a nut member (2) and accordingly a rotation resistance acting between a screw shaft (1) and the nut member (2) can be reduced. A pair of ball introduction passages (25), which have an inner diameter larger than the diameter of the balls and are continuously connected to both ends of the loaded rolling groove (21), are drilled in the peripheral wall of the nut member (2). The ball introduction passages (25) are formed to be passed through the nut member (2) in the tangential direction of a shaft hole (20). A circulation passage forming member (4) forms a no-load return passage (42) for the balls by connecting openings of the paired ball introduction passages (25) on the side opposite to the loaded rolling groove (21) to each other.</p>
