To expand use by particularly imparting high extraction force to a bolt.

This bolt cap fastening structure is constituted so that the bolt 2 has an upper end head part 20, a large diameter washer 22 arranged around a shaft part and a fitting neck part 24 or 24a on the washer, a cap 1 has a cap part 10 and a clamp part 18, and the clamp part is fitted to the neck part, and the cap part is crowned on the head part. The clamp part 18 is formed in a substantially C shape insertable from the radial direction into the neck part, and has a pair of extending piece parts 19 projected forward from both ends of the C shape.