A ball screw device comprising: a screw shaft including a shaft-side ball thread groove having a spiral shape on an outer-circumferential surface thereof, the screw shaft rotating during use, a nut (3, 3a) including a nut-side ball thread groove having a spiral shape on an inner-circumferential surface thereof, the nut (3, 3a) moving linearly during use, balls arranged between the shaft-side ball thread groove and the nut-side ball thread groove, a fitting cylinder fitted and fixed to the nut and linearly moving with the nut, a housing having an insertion hole through which the nut can be inserted in an axial direction, and an anti-rotation member (7, 7a) preventing the nut from rotating relative to the housing, a first member, which is one of the nut and the fitting cylinder, having a retaining concave portion (19, 19a, 19b) on an outer-circumferential surface thereof, the retaining concave portion (19, 19a, 19b) capable of engaging with a radially inner portion of the anti-rotation member (7, 7a) in a circumferential direction and including a closed surface facing the axial direction, and the insertion hole having a guide concave groove (24, 24a, 24b) on an inner-circumferential surface thereof, the guide concave groove (24, 24a, 24b) capable of engaging with a radially outer portion of the anti-rotation member (7, 7a) in the circumferential direction and extending in the axial direction, characterized in that the radially inner portion of the anti-rotation member (7, 7a) arranged inside the retaining concave portion (19, 19a, 19b) with the radially inner portion held between the closed surface and an end surface (3x, 5x) in the axial direction of a second member, which is the other of the nut (3, 3a) and the fitting cylinder, and the radially outer portion of the anti-rotation member (7, 7a) arranged inside the guide concave groove (24, 24a, 24b) so as to slide in the axial direction. a screw shaft including a shaft-side ball thread groove having a spiral shape on an outer-circumferential surface thereof, the screw shaft rotating during use, a nut (3, 3a) including a nut-side ball thread groove having a spiral shape on an inner-circumferential surface thereof, the nut (3, 3a) moving linearly during use, balls arranged between the shaft-side ball thread groove and the nut-side ball thread groove, a fitting cylinder fitted and fixed to the nut and linearly moving with the nut, a housing having an insertion hole through which the nut can be inserted in an axial direction, and an anti-rotation member (7, 7a) preventing the nut from rotating relative to the housing, a first member, which is one of the nut and the fitting cylinder, having a retaining concave portion (19, 19a, 19b) on an outer-circumferential surface thereof, the retaining concave portion (19, 19a, 19b) capable of engaging with a radially inner portion of the anti-rotation member (7, 7a) in a circumferential direction and including a closed surface facing the axial direction, and the insertion hole having a guide concave groove (24, 24a, 24b) on an inner-circumferential surface thereof, the guide concave groove (24, 24a, 24b) capable of engaging with a radially outer portion of the anti-rotation member (7, 7a) in the circumferential direction and extending in the axial direction, characterized in that the radially inner portion of the anti-rotation member (7, 7a) arranged inside the retaining concave portion (19, 19a, 19b) with the radially inner portion held between the closed surface and an end surface (3x, 5x) in the axial direction of a second member, which is the other of the nut (3, 3a) and the fitting cylinder, and the radially outer portion of the anti-rotation member (7, 7a) arranged inside the guide concave groove (24, 24a, 24b) so as to slide in the axial direction.