As compared to a single-axis actuator in which a ball returning passage of a ball screw mechanism is made of a circulating tube, the cross-sectional shape perpendicular to the longitudinal direction of a threaded shaft of a slider can be made small, when the diameter of the outer circumferential circle of the threaded shaft of the present invention is same as that of the circulating tube type, and the load capacity can be made large, when the size in the longitudinal direction of the threaded shaft of the slider of the present invention is same as that of the circulating tube type. The ball returning passage for returning the balls (4) of a ball screw mechanism from an end point to a start point of a raceway encompasses: a through hole (24) penetrating through a slider (2) in a longitudinal direction of a threaded shaft (3); and end deflectors (6). Each of the end deflector (6) has a direction changing passage to be connected with the through hole (24). A main body (61) of the end deflector (6) is fit into a notch part (26) arranged on each of both ends in the longitudinal direction of the threaded shaft (3) of a slider body (2A).