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 including a nut-side ball thread groove having a spiral shape on an inner-circumferential surface thereof, the nut 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 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 on an outer-circumferential surface thereof, the retaining concave portion capable of engaging with a radially inner portion of the anti-rotation member in a circumferential direction and including a closed surface facing the axial direction, and the insertion hole having a guide concave groove on an inner-circumferential surface thereof, the guide concave groove capable of engaging with a radially outer portion of the anti-rotation member in the circumferential direction and extending in the axial direction, characterized in that the radially inner portion of the anti-rotation member arranged inside the retaining concave portion with the radially inner portion held between the closed surface and an end surface in the axial direction of a second member, which is the other of the nut and the fitting cylinder, and the radially outer portion of the anti-rotation member arranged inside the guide concave groove 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 including a nut-side ball thread groove having a spiral shape on an inner-circumferential surface thereof, the nut 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 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 on an outer-circumferential surface thereof, the retaining concave portion capable of engaging with a radially inner portion of the anti-rotation member in a circumferential direction and including a closed surface facing the axial direction, and the insertion hole having a guide concave groove on an inner-circumferential surface thereof, the guide concave groove capable of engaging with a radially outer portion of the anti-rotation member in the circumferential direction and extending in the axial direction, characterized in that the radially inner portion of the anti-rotation member arranged inside the retaining concave portion with the radially inner portion held between the closed surface and an end surface in the axial direction of a second member, which is the other of the nut and the fitting cylinder, and the radially outer portion of the anti-rotation member arranged inside the guide concave groove so as to slide in the axial direction. <EOS>