Ball screw device and steering system

A ball screw device comprising:

• a ball screw shaft having a ball rolling groove disposed on an outer peripheral surface of the ball screw shaft in a helical pattern;
• a ball screw nut having a ball rolling groove disposed on an inner peripheral surface of the ball screw nut in a helical pattern;
• a plurality of balls provided between the ball rolling groove of the ball screw shaft and the ball rolling groove of the ball screw nut;
• and
• a retainer arranged between the ball screw shaft and the ball screw nut, the retainer having a plurality of retainer grooves configured to retain the plurality of balls in a rollable manner, each retainer groove of the plurality of retainer grooves having (i) a straight portion extending in an axial direction of the ball screw shaft and (ii) a chamfer portion of a wall of each retainer groove in the axial direction, which intersects with the straight portion, wherein:
• a distance between a first end of the ball screw nut and a first end of the retainer, which is one of two ends of the retainer that is closer to the first end of the ball screw nut, is set equal to or larger than a distance between a terminal end of the retainer groove that is closest to a second end of the retainer and a part of a last ball of the plurality of balls that is closest to a second end of the ball screw nut.