A ball screw includes: a screw shaft (10) having a spiral first screw groove (11) formed in an outer peripheral surface thereof; a nut (20) disposed on the periphery of the screw shaft (10) and having a spiral second screw groove (21) formed in an inner peripheral surface thereof; a plurality of balls (30) accommodated in a rolling path (23) formed by the opposing screw grooves (11, 21); and a circulation piece (40) that forms a ball return passage (42) for circulating the plurality of balls (30) through one circuit or less of the rolling path (23). Traffic variation is made to be small by determining the maximum angle of inclination (a) of the ball return passage (42) in an expression using the lead (L), the ball diameter (Da), and the lead angle (β).