In a ball screw mechanism, a retainer is provided between an outer circumference of a ball screw shaft and an inner circumference of a ball nut and has a plurality of retainer grooves arranged in the circumferential direction for retaining a plurality of rolling balls. Each retainer groove of the retainer takes a form that allows the rolling balls therein to move radially outward but restrains the rolling balls therein from moving radially inward. Therefore, in the ball screw mechanism and an electric power steering device incorporating the same, the movement of the retainer in radial directions can be restricted by the plurality of rolling balls which are rolled along ball rolling grooves spirally formed respectively on the outer circumference of the ball screw shaft and the inner circumference of the ball nut.