A cap is applied to a securing structure part for securing structure elements using a fastener passed through through-holes formed in the structure elements, and a collar fastened to the tip part of the fastener. A filler engaging part for engaging a cured filler, configured with a female thread that turns about a center axis, is formed on the inner peripheral surface of the cap. The filler engaging part includes a first helical engagement part formed in the region near the cap inner part of the inner peripheral surface in the center axis direction; and a second helical engagement part formed in the region near the cap opening-end part in the center axis direction, the inner diameter of the second helical engagement part being greater than that of the first helical engagement part, and having a helical direction that is opposite to that of the first helical engagement part.