To remove metal easily from a hammer case, make fractional recovery possible and reuse an expensive hammer case.

A taper part and a thread part are provided at the peripheral part of metal 2. A cap 9 is screwed with the thread part of the metal 2, and a hammer case 5 is provided with the inner surface coinciding with the taper part of the metal 2. The taper part of the metal 2 is thereby fixed rigidly to the inner surface of the hammer case 5 by wedge action by fastening the cap 9.