A ball screw device comprising: a threaded shaft having an outer peripheral face in which a groove is formed; a ball nut fitted onto the threaded shaft and having an inner peripheral face in which a groove is formed; a plurality of balls rollably disposed in a spiral ball rolling path formed by the groove of the ball nut and the groove of the threaded shaft; and a cylinder disposed so as to surround an outer periphery of the ball nut, wherein in the ball rolling path, accommodation recesses that pass through a peripheral wall of the ball nut in a thickness direction are formed in at least two accommodation recess formed positions that are apart from each other in an axial direction of the threaded shaft, in an outer peripheral face of the ball nut, an outer periphery turning groove that turns in a spiral manner along the outer periphery of the ball nut is formed, and the outer periphery turning groove and an inner peripheral face of the cylinder constitute a turning rolling path in which the balls are rollable, the ball screw device further comprises: deflectors accommodated in the respective accommodation recesses, and each having a connection passage that connects a corresponding one of the accommodation recess formed positions to the turning rolling path; and a relative rotation prohibiting structure that prohibits rotation of the cylinder relative to the ball nut, and the turning rolling path and the two connection passages constitute a circulation path through which the balls are returned from one of the accommodation recess formed positions to the other one of the accommodation recess formed positions. a threaded shaft having an outer peripheral face in which a groove is formed; a ball nut fitted onto the threaded shaft and having an inner peripheral face in which a groove is formed; a plurality of balls rollably disposed in a spiral ball rolling path formed by the groove of the ball nut and the groove of the threaded shaft; and a cylinder disposed so as to surround an outer periphery of the ball nut, wherein in the ball rolling path, accommodation recesses that pass through a peripheral wall of the ball nut in a thickness direction are formed in at least two accommodation recess formed positions that are apart from each other in an axial direction of the threaded shaft, in an outer peripheral face of the ball nut, an outer periphery turning groove that turns in a spiral manner along the outer periphery of the ball nut is formed, and the outer periphery turning groove and an inner peripheral face of the cylinder constitute a turning rolling path in which the balls are rollable, the ball screw device further comprises: deflectors accommodated in the respective accommodation recesses, and each having a connection passage that connects a corresponding one of the accommodation recess formed positions to the turning rolling path; and a relative rotation prohibiting structure that prohibits rotation of the cylinder relative to the ball nut, and deflectors accommodated in the respective accommodation recesses, and each having a connection passage that connects a corresponding one of the accommodation recess formed positions to the turning rolling path; and a relative rotation prohibiting structure that prohibits rotation of the cylinder relative to the ball nut, and the turning rolling path and the two connection passages constitute a circulation path through which the balls are returned from one of the accommodation recess formed positions to the other one of the accommodation recess formed positions.