To enable easy position adjustment of a lip of a seal with a screw axis when a contact seal is attached to an end part in the axial direction of a nut of a ball screw.

A seal 4 is attached to a nut 1 by fixing a seal cap 5 where a seal 4 is fixed to an attachment part 12 of the nut. Three plane parts 13 are formed at different positions in a peripheral direction of the outer peripheral face of the attachment part 12. The seal cap 5 is composed of an annular part 51 fitting onto the attachment part 12, and a flange part 52 facing inward. In the annular part 51, at the positions same as three plane parts 13, three through screw holes (through holes) 51a are formed. The annular part 51 of the seal cap 5 is fitted onto the attachment part 12 of the nut 1 from outside, and the planar part 13 is matched to the through screw hole 51a to screw with a small screw 7. Then, the seal 4 is attached to the nut 1, and the position adjustment between the lip 41 of the seal 4 and the screw axis 2 in the periphery direction is performed.