A ball screw device comprising: a ball screw shaft having an outer peripheral surface on which a ball rolling groove is spirally formed; a ball screw nut having an inner peripheral surface on which a ball rolling groove is spirally formed; 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 disposed between the ball screw shaft and the ball screw nut and having a cylindrical shape in which a plurality of retainer grooves that retain the balls in a rollable manner and a plurality of separating portions each of which is a portion between the retainer grooves adjacent to each other in a circumferential direction and separates the balls from each other in the circumferential direction, wherein the retainer groove each have an elongated-hole shape that extends inclined at a certain angle with respect to the axis of the rack shaft, wherein each retainer groove forms a right angle with the ball rolling groove of the rack shaft and with the ball rolling groove of the ball screw nut, characterized in that the retainer has at least one structure of a structure A in which both end portions of each retainer groove in an extending direction of the retainer groove has a shape that extends more outward than a circular-arc surface shape in a direction orthogonal to the extending direction, a structure B in which a central portion positioned between both base portions that are both end portions of each separating portion in the extending direction of the retainer groove has a length longer in at least one of a radial direction and a circumferential direction of the retainer than a length of the base portions, and a structure C in which a groove is formed in a portion other than the central portion of each separating portion in the retainer. a ball screw shaft having an outer peripheral surface on which a ball rolling groove is spirally formed; a ball screw nut having an inner peripheral surface on which a ball rolling groove is spirally formed; 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 disposed between the ball screw shaft and the ball screw nut and having a cylindrical shape in which a plurality of retainer grooves that retain the balls in a rollable manner and a plurality of separating portions each of which is a portion between the retainer grooves adjacent to each other in a circumferential direction and separates the balls from each other in the circumferential direction, wherein the retainer groove each have an elongated-hole shape that extends inclined at a certain angle with respect to the axis of the rack shaft, wherein each retainer groove forms a right angle with the ball rolling groove of the rack shaft and with the ball rolling groove of the ball screw nut, characterized in that the retainer has at least one structure of a structure A in which both end portions of each retainer groove in an extending direction of the retainer groove has a shape that extends more outward than a circular-arc surface shape in a direction orthogonal to the extending direction, a structure B in which a central portion positioned between both base portions that are both end portions of each separating portion in the extending direction of the retainer groove has a length longer in at least one of a radial direction and a circumferential direction of the retainer than a length of the base portions, and a structure C in which a groove is formed in a portion other than the central portion of each separating portion in the retainer. a structure A in which both end portions of each retainer groove in an extending direction of the retainer groove has a shape that extends more outward than a circular-arc surface shape in a direction orthogonal to the extending direction, a structure B in which a central portion positioned between both base portions that are both end portions of each separating portion in the extending direction of the retainer groove has a length longer in at least one of a radial direction and a circumferential direction of the retainer than a length of the base portions, and a structure C in which a groove is formed in a portion other than the central portion of each separating portion in the retainer.