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 groove of the ball nut, accommodation recesses that pass through a peripheral wall of the ball nut in a thickness direction of the ball nut are formed in two accommodation recess formed positions (47A, 47B) that are apart from each other in an axial direction of the threaded shaft, in at least one of an outer peripheral face of the ball nut and an inner peripheral face of the cylinder, a turning groove that turns in a spiral manner along a corresponding one of the outer peripheral face of the ball nut and the inner peripheral face of the cylinder is formed, and the turning groove and the outer peripheral face of the ball nut or the 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 the ball rolling path and the turning rolling path to each other; the two connection passages and the turning rolling path constitute a returning path through which the balls are returned from one of the two accommodation recess formed positions (47A, 47B) to the other one of the two accommodation recess formed positions (47A, 47B), the ball nut has an internally-fitting portion having an outer peripheral face formed such that a distance between the outer peripheral face and a central axis of the ball nut is non-uniform along a circumferential direction of the ball nut, the internally-fitting portion being a part of the ball nut in an axial direction of the ball nut, characterized in that relative rotation between the ball nut and the cylinder is prevented by fitting an outer periphery of the internally-fitting portion and an inner periphery of the cylinder to each other and the ball screw device further comprises an axial movement prevention structure (36, 37) that prevents an axial movement of the cylinder relative to the ball nut. 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 groove of the ball nut, accommodation recesses that pass through a peripheral wall of the ball nut in a thickness direction of the ball nut are formed in two accommodation recess formed positions (47A, 47B) that are apart from each other in an axial direction of the threaded shaft, in at least one of an outer peripheral face of the ball nut and an inner peripheral face of the cylinder, a turning groove that turns in a spiral manner along a corresponding one of the outer peripheral face of the ball nut and the inner peripheral face of the cylinder is formed, and the turning groove and the outer peripheral face of the ball nut or the 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 the ball rolling path and the turning rolling path to each other; the two connection passages and the turning rolling path constitute a returning path through which the balls are returned from one of the two accommodation recess formed positions (47A, 47B) to the other one of the two accommodation recess formed positions (47A, 47B), the ball nut has an internally-fitting portion having an outer peripheral face formed such that a distance between the outer peripheral face and a central axis of the ball nut is non-uniform along a circumferential direction of the ball nut, the internally-fitting portion being a part of the ball nut in an axial direction of the ball nut, characterized in that relative rotation between the ball nut and the cylinder is prevented by fitting an outer periphery of the internally-fitting portion and an inner periphery of the cylinder to each other and the ball screw device further comprises an axial movement prevention structure (36, 37) that prevents an axial movement of the cylinder relative to the ball nut.