A ball screw device comprising: a seal having a non circular inner peripheral surface; a threaded shaft extending in an axis direction and having a helical rolling groove provided in an outer peripheral surface, and a cylindrical shaped nut having a rolling groove, which faces the rolling groove, in an inner peripheral surface, and screwed into the threaded shaft through a plurality of balls arranged between the rolling groove and the rolling groove of the threaded shaft, the seal for a ball screw device comprising: a seal member provided on an inner diameter side of the nut, which seals a gap between the nut and the threaded shaft in both end parts of the nut in the axis direction by sliding contact of an inner peripheral edge part with an outer peripheral surface of the threaded shaft; and a guide part which is provided adjacent to the inner peripheral edge of the seal member and being provided in at least one of end surfaces of the seal member in the axis direction, which has projections or recessed parts in the axis direction in a portion corresponding to the inner peripheral edge part that is in sliding contact with the rolling groove of the threaded shaft or a land part between the rolling grooves (11, 21), characterized in that the guide part is provided in a shape projecting or recessed in the axis direction and a size of the guide part in a radial direction gradually increases and decreases at least at 240° or smaller in the circumferential direction of the seal member, the guide part being used for matching the phases of the threaded shaft and the seal member. a seal having a non circular inner peripheral surface; a threaded shaft extending in an axis direction and having a helical rolling groove provided in an outer peripheral surface, and a cylindrical shaped nut having a rolling groove, which faces the rolling groove, in an inner peripheral surface, and screwed into the threaded shaft through a plurality of balls arranged between the rolling groove and the rolling groove of the threaded shaft, the seal for a ball screw device comprising: a seal member provided on an inner diameter side of the nut, which seals a gap between the nut and the threaded shaft in both end parts of the nut in the axis direction by sliding contact of an inner peripheral edge part with an outer peripheral surface of the threaded shaft; and a guide part which is provided adjacent to the inner peripheral edge of the seal member and being provided in at least one of end surfaces of the seal member in the axis direction, which has projections or recessed parts in the axis direction in a portion corresponding to the inner peripheral edge part that is in sliding contact with the rolling groove of the threaded shaft or a land part between the rolling grooves (11, 21), characterized in that the guide part is provided in a shape projecting or recessed in the axis direction and a size of the guide part in a radial direction gradually increases and decreases at least at 240° or smaller in the circumferential direction of the seal member, the guide part being used for matching the phases of the threaded shaft and the seal member. a seal member provided on an inner diameter side of the nut, which seals a gap between the nut and the threaded shaft in both end parts of the nut in the axis direction by sliding contact of an inner peripheral edge part with an outer peripheral surface of the threaded shaft; and a guide part which is provided adjacent to the inner peripheral edge of the seal member and being provided in at least one of end surfaces of the seal member in the axis direction, which has projections or recessed parts in the axis direction in a portion corresponding to the inner peripheral edge part that is in sliding contact with the rolling groove of the threaded shaft or a land part between the rolling grooves (11, 21), characterized in that the guide part is provided in a shape projecting or recessed in the axis direction and a size of the guide part in a radial direction gradually increases and decreases at least at 240° or smaller in the circumferential direction of the seal member, the guide part being used for matching the phases of the threaded shaft and the seal member.