A ball screw device comprising: a ball screw shaft having outer-peripheral rolling grooves formed in a spiral manner on an outer peripheral surface; a ball screw nut arranged on the same axis on an outer side in a radial direction with respect to the ball screw shaft and formed in a cylindrical shape so as to rotate relatively with respect to the ball screw shaft , in which inner-peripheral rolling grooves are formed in a spiral manner on an inner peripheral surface; balls capable of rolling in a rolling path surrounded by the outer-peripheral rolling grooves and the inner-peripheral rolling grooves ; and a circulation path allowing one end of the rolling path to communicate with the other end thereof to circulate the balls endlessly, wherein the ball screw nut includes a nut main body in which at least part of the inner-peripheral rolling grooves and at least part of the circulation path are respectively formed and a deflector mounting hole piercing in the radial direction is provided, and a deflector inserted into the deflector mounting hole from the outer side in the radial direction and fixed thereto, in which a passage allowing the inner-peripheral rolling grooves of the nut main body to communicate with the circulation path to allow the balls to pass therethrough is formed and an open groove allowing the passage to open to a side surface of the circulation path side is formed, and characterized in that at least one of the nut main body and the deflector has an inclined portion provided at a peripheral part of a connection portion where the circulation path of the nut main body and the passage of the deflector are connected to each other, the inclined portion secures passing of the balls at the connection portion when an insertion position of the deflector with respect to the deflector mounting hole is displaced from a desired position within a range of a given distance. a ball screw shaft having outer-peripheral rolling grooves formed in a spiral manner on an outer peripheral surface; a ball screw nut arranged on the same axis on an outer side in a radial direction with respect to the ball screw shaft and formed in a cylindrical shape so as to rotate relatively with respect to the ball screw shaft , in which inner-peripheral rolling grooves are formed in a spiral manner on an inner peripheral surface; balls capable of rolling in a rolling path surrounded by the outer-peripheral rolling grooves and the inner-peripheral rolling grooves ; and a circulation path allowing one end of the rolling path to communicate with the other end thereof to circulate the balls endlessly, wherein the ball screw nut includes a nut main body in which at least part of the inner-peripheral rolling grooves and at least part of the circulation path are respectively formed and a deflector mounting hole piercing in the radial direction is provided, and a deflector inserted into the deflector mounting hole from the outer side in the radial direction and fixed thereto, in which a passage allowing the inner-peripheral rolling grooves of the nut main body to communicate with the circulation path to allow the balls to pass therethrough is formed and an open groove allowing the passage to open to a side surface of the circulation path side is formed, and characterized in that at least one of the nut main body and the deflector has an inclined portion provided at a peripheral part of a connection portion where the circulation path of the nut main body and the passage of the deflector are connected to each other, the inclined portion secures passing of the balls at the connection portion when an insertion position of the deflector with respect to the deflector mounting hole is displaced from a desired position within a range of a given distance. <EOS>