A ball screw device comprising: a screw shaft having an outer peripheral surface provided with a helical outer peripheral rolling groove; a nut formed in a tubular shape, having an inner peripheral surface provided with a helical inner peripheral rolling groove, disposed on an outer peripheral side of the screw shaft, and having mounting holes (41, 42) extending between two different single-turn rolling grooves (21a1, 21a6) of single-turn rolling grooves (21al to 21a6) aligned in an axial direction thereof and an outer peripheral surface thereof, the inner peripheral rolling groove being defined as a collection of the single-turn rolling grooves aligned in the axial direction and each having a single turn about an axis thereof; rolling elements arranged in a rollable manner in a rolling path defined between the outer peripheral rolling groove and the inner peripheral rolling groove; a return path that short-circuits between openings (41a, 42a) of the mounting holes (41, 42) which are open to the rolling path and respectively formed in the two different single-turn rolling grooves (21a1, 21a6), and that allows the rolling elements rolling in the rolling path to circulate endlessly; and a deflector (51, 52) that is accommodated and fixed in the mounting hole (41, 42), that defines a part of or the entire return path therein, and that guides the rolling elements rolling in the rolling path to the return path and returns the rolling elements to the rolling path; characterized in that the inner peripheral rolling groove of the nut includes, in a range between the openings (41a, 42a) of the mounting holes (41, 42), a constant pitch circle diameter region (CoA) spaced apart from the openings (41a, 42a) and formed such that a pitch circle diameter of the rolling path is constant, and gradually-changing pitch circle diameter regions (ChA) formed such that the pitch circle diameter gradually increases from the constant pitch circle diameter region (CoA), in ranges from opposite ends of the constant pitch circle diameter region to the respective openings (41a, 42a); and at least the constant pitch circle diameter region (CoA) has a surface hardness greater than or equal to a predetermined value, among the constant pitch circle diameter region (CoA) and the gradually-changing pitch circle diameter regions (ChA) excluding edges (41b, 42b) at boundaries between the respective openings (41a, 42a) and the inner peripheral rolling groove, and the edges at the boundaries between the respective openings and the inner peripheral rolling groove have a surface hardness less than the predetermined value. a screw shaft having an outer peripheral surface provided with a helical outer peripheral rolling groove; a nut formed in a tubular shape, having an inner peripheral surface provided with a helical inner peripheral rolling groove, disposed on an outer peripheral side of the screw shaft, and having mounting holes (41, 42) extending between two different single-turn rolling grooves (21a1, 21a6) of single-turn rolling grooves (21al to 21a6) aligned in an axial direction thereof and an outer peripheral surface thereof, the inner peripheral rolling groove being defined as a collection of the single-turn rolling grooves aligned in the axial direction and each having a single turn about an axis thereof; rolling elements arranged in a rollable manner in a rolling path defined between the outer peripheral rolling groove and the inner peripheral rolling groove; a return path that short-circuits between openings (41a, 42a) of the mounting holes (41, 42) which are open to the rolling path and respectively formed in the two different single-turn rolling grooves (21a1, 21a6), and that allows the rolling elements rolling in the rolling path to circulate endlessly; and a deflector (51, 52) that is accommodated and fixed in the mounting hole (41, 42), that defines a part of or the entire return path therein, and that guides the rolling elements rolling in the rolling path to the return path and returns the rolling elements to the rolling path; characterized in that the inner peripheral rolling groove of the nut includes, in a range between the openings (41a, 42a) of the mounting holes (41, 42), a constant pitch circle diameter region (CoA) spaced apart from the openings (41a, 42a) and formed such that a pitch circle diameter of the rolling path is constant, and gradually-changing pitch circle diameter regions (ChA) formed such that the pitch circle diameter gradually increases from the constant pitch circle diameter region (CoA), in ranges from opposite ends of the constant pitch circle diameter region to the respective openings (41a, 42a); and at least the constant pitch circle diameter region (CoA) has a surface hardness greater than or equal to a predetermined value, among the constant pitch circle diameter region (CoA) and the gradually-changing pitch circle diameter regions (ChA) excluding edges (41b, 42b) at boundaries between the respective openings (41a, 42a) and the inner peripheral rolling groove, and the edges at the boundaries between the respective openings and the inner peripheral rolling groove have a surface hardness less than the predetermined value.