A ball screw mechanism including a deflector, a screw shaft, and a nut, a ball rolling passage being defined by a thread groove formed on an outer circumferential surface of the screw shaft and a thread groove formed on an inner circumferential surface of the nut, the deflector being configured to be fitted into the nut to guide a plurality of balls from the ball rolling passage to a ball return passage formed in the nut, the deflector comprising: a deflector main body having a tongue extending toward the ball rolling passage for scooping up the balls from the ball rolling passage, and a main body side ball guide groove for defining a part of a ball guide passage for guiding the balls scooped up by the tongue to the ball return passage, the deflector main body being configured to be fixedly secured to a prescribed position of the nut by engaging with a part of the nut; and a component piece having a component piece side ball guide groove defining a remaining part of the ball guide passage to define the ball guide passage jointly with the main body side ball guide groove, the component piece being configured to be fitted to a prescribed position of the deflector main body by engaging with a part of the deflector main body, the deflector main body and the component piece being configured to be joined in an axial direction of the nut, characterized in that the deflector is configured to be received in a receiving hole opening out at an outer circumferential surface of the nut from radially outside of the nut, and the deflector is provided with a pair of mutually parallel side surfaces facing in the axial direction of the nut, and one of the side surfaces is defined solely by the deflector main body while the other side surface is jointly defined by the deflector main body and the component piece as a flush surface such that the component piece is held immobile in the axial direction of the nut by being interposed between the nut and the deflector main body in the receiving hole. a deflector main body having a tongue extending toward the ball rolling passage for scooping up the balls from the ball rolling passage, and a main body side ball guide groove for defining a part of a ball guide passage for guiding the balls scooped up by the tongue to the ball return passage, the deflector main body being configured to be fixedly secured to a prescribed position of the nut by engaging with a part of the nut; and a component piece having a component piece side ball guide groove defining a remaining part of the ball guide passage to define the ball guide passage jointly with the main body side ball guide groove, the component piece being configured to be fitted to a prescribed position of the deflector main body by engaging with a part of the deflector main body, the deflector main body and the component piece being configured to be joined in an axial direction of the nut, characterized in that the deflector is configured to be received in a receiving hole opening out at an outer circumferential surface of the nut from radially outside of the nut, and the deflector is provided with a pair of mutually parallel side surfaces facing in the axial direction of the nut, and one of the side surfaces is defined solely by the deflector main body while the other side surface is jointly defined by the deflector main body and the component piece as a flush surface such that the component piece is held immobile in the axial direction of the nut by being interposed between the nut and the deflector main body in the receiving hole.