The invention provides a ball screw device and a steering system including the ball screw device. Provided is a ball screw device in which a deflector can securely be prevented from falling into a nutwhen the deflector is attached. A ball screw device (40) includes a ball screw shaft (20), a nut (21), rolling elements (24), a recirculation path (arc surface 2), and a deflector (51). An attachmenthole (41) has a pair of first faces (411a, 411b). The deflector (51) has a pair of second faces (511a, 511b) that respectively face the pair of first faces in a state in which the deflector (51) is housed in the attachment hole. The first faces of the attachment hole and the second faces of the deflector have a locking structure in which the first faces of the attachment hole and the second facesof the deflector respectively lock each other in an improper state in which a part of the deflector is inserted into the attachment hole in a posture different from a posture of the deflector housedin the attachment hole.