A ball screw device (1, 1a) comprising: a screw shaft having a shaft-side ball thread groove with a spiral shape on an outer circumferential surface thereof; a nut having a nut-side ball thread groove with a spiral shape on an inner circumferential surface thereof, the shaft-side ball thread groove and the nut-side ball thread groove constituting a load passage having a spiral shape; a circulation part (5, 5a) attached to an outer circumferential surface of the nut, the circulation part (5, 5a) forming a circulation passage connecting a starting point and an end point of the load passage between the circulation part (5, 5a) and the nut; and a plurality of balls arranged in the load passage and the circulation passage so as to be able to roll; the nut having a housing concave groove formed on the outer circumferential surface thereof, and including a groove bottom surface having a flat surface shape and a pair of groove wall surfaces facing each other in a circumferential direction thereof, and a pair of through holes distantly positioned in the groove bottom surface and opening thereto and penetrating the nut in a radial direction thereof; and the circulation part (5, 5a) being arranged inside the housing concave groove, and having a pair of leg portions inserted into the pair of through holes, and a pair of fixing portions (17, 17a), characterized in that the housing concave groove linearly extends in an axial direction so as to open to end surfaces on both outer sides in the axial direction of the nut; the pair of groove wall surfaces has an engaging recess portion in an end portion on each of both outer sides in the axial direction; and the pair of fixing portions (17, 17a) is provided in end portions on both outer sides in the axial direction of the circulation part (5, 5a); and each fixing portion of the pair of fixing portions (17, 17a) has an engaging convex portion configured to engage with the engaging recess portion so that the circulation part (5, 5a) is not able to displace toward an outer side in the radial direction and displace in the circumferential direction with respect to the nut. a screw shaft having a shaft-side ball thread groove with a spiral shape on an outer circumferential surface thereof; a nut having a nut-side ball thread groove with a spiral shape on an inner circumferential surface thereof, the shaft-side ball thread groove and the nut-side ball thread groove constituting a load passage having a spiral shape; a circulation part (5, 5a) attached to an outer circumferential surface of the nut, the circulation part (5, 5a) forming a circulation passage connecting a starting point and an end point of the load passage between the circulation part (5, 5a) and the nut; and a plurality of balls arranged in the load passage and the circulation passage so as to be able to roll; the nut having a housing concave groove formed on the outer circumferential surface thereof, and including a groove bottom surface having a flat surface shape and a pair of groove wall surfaces facing each other in a circumferential direction thereof, and a pair of through holes distantly positioned in the groove bottom surface and opening thereto and penetrating the nut in a radial direction thereof; and the circulation part (5, 5a) being arranged inside the housing concave groove, and having a pair of leg portions inserted into the pair of through holes, and a pair of fixing portions (17, 17a), characterized in that the housing concave groove linearly extends in an axial direction so as to open to end surfaces on both outer sides in the axial direction of the nut; the pair of groove wall surfaces has an engaging recess portion in an end portion on each of both outer sides in the axial direction; and the pair of fixing portions (17, 17a) is provided in end portions on both outer sides in the axial direction of the circulation part (5, 5a); and each fixing portion of the pair of fixing portions (17, 17a) has an engaging convex portion configured to engage with the engaging recess portion so that the circulation part (5, 5a) is not able to displace toward an outer side in the radial direction and displace in the circumferential direction with respect to the nut.