A ball screw mechanism comprising: a screw shaft, on an outer peripheral surface which a spiral screw shaft-side rolling groove is formed; a cylindrical nut, on an inner peripheral surface of which a spiral nut-side rolling groove that faces the screw shaft-side rolling groove is formed; and a plurality of balls disposed in a rolling passage that is a space surrounded by the screw shaft-side rolling grove and the nut-side rolling groove, wherein the nut is provided with: shift portions provided at both ends of the nut-side rolling groove to shift a track of the nut-side rolling groove in an axial direction of the screw shaft from the screw shaft-side rolling groove and to displace the nut-side rolling groove to be deeper in a radial direction of the screw shaft as the track of the nut-side rolling groove is shifted; a circulation passage that connects between the shift portions provided at both ends of the nut-side rolling groove; and passage entrances that are each an entrance of a passage that connects between the shift portion and the circulation passage, characterized in that the passage entrances are each positioned, in the axial direction of the screw shaft, between a starting position of the shift portion and a position facing an outermost periphery of a screw thread that is adjacent to the screw shaft-side rolling groove which faces the starting position of the shift portion. a screw shaft, on an outer peripheral surface which a spiral screw shaft-side rolling groove is formed; a cylindrical nut, on an inner peripheral surface of which a spiral nut-side rolling groove that faces the screw shaft-side rolling groove is formed; and a plurality of balls disposed in a rolling passage that is a space surrounded by the screw shaft-side rolling grove and the nut-side rolling groove, wherein the nut is provided with: shift portions provided at both ends of the nut-side rolling groove to shift a track of the nut-side rolling groove in an axial direction of the screw shaft from the screw shaft-side rolling groove and to displace the nut-side rolling groove to be deeper in a radial direction of the screw shaft as the track of the nut-side rolling groove is shifted; a circulation passage that connects between the shift portions provided at both ends of the nut-side rolling groove; and passage entrances that are each an entrance of a passage that connects between the shift portion and the circulation passage, characterized in that shift portions provided at both ends of the nut-side rolling groove to shift a track of the nut-side rolling groove in an axial direction of the screw shaft from the screw shaft-side rolling groove and to displace the nut-side rolling groove to be deeper in a radial direction of the screw shaft as the track of the nut-side rolling groove is shifted; a circulation passage that connects between the shift portions provided at both ends of the nut-side rolling groove; and passage entrances that are each an entrance of a passage that connects between the shift portion and the circulation passage, characterized in that the passage entrances are each positioned, in the axial direction of the screw shaft, between a starting position of the shift portion and a position facing an outermost periphery of a screw thread that is adjacent to the screw shaft-side rolling groove which faces the starting position of the shift portion.