To install a cisclip in a ring groove of a work even in the case when an outside diameter split port end surfaces which are made contact with each other is larger than an inside diameter of a hole of the work by arranging one of the split port end surfaces out of an extension region of the other split port end surface in the case when an end surface of the cisclip agrees with a pressure surface.

Roughly the whole length of an insertion part 64 of a punch 60 is divided into four parts as a central hole 74 is formed along a shaft center from a head end surface of a tapered part 66 and a cross slit 76 including a starting end and a final end of a pressure surface 72 is formed. As an upper end surface of a cisclip comes to be in a state to be pressurized on the pressure surface 72 by friction with an inner peripheral surface 42c of a tapered guide hole 42 while the punch 60 is inserted into the tapered guide hole 42, and accordingly, in case of applying the punch 60 the pressure surface 72 of which is constituted spiral the cisclip at the time of passing the tapered guide hole 42 is deformed spiral along the pressure surface 72, and one split port end surface is arranged out of an extension region of the other split port end surface.