To provide a cap for a nut capable of performing adjustment by a washer so that shearing action is performed in a straight part of a bolt when fastening the bolt with the nut.

In this cap 1 for the nut covering a part of a side surface and a top surface of the nut, a hole 1a for bolt insertion is provided on a top surface of the cap, a rotation stop hole 1c for exposing a corner part 2a of the nut 2 is provided on a side wall part 1b, and a hole part 1e communicating from the rotation stop hole 1c and avoiding a part of the washer 3 for level adjustment is provided on a fixing part 1d.