The guide rods 25, 26 extending straight along the discharge port 17 are fixed to the contact member 24, and this contact member 24 is retained slidably on the nose 16 via the guide rods 25, 26. The nose extension 23 is formed at the free end of the discharge port 17 so as to be integral with the nose 16 so that the nose extension covers the outer circumferential surface of the discharge port 17. The contact member 24 is held in the annular space formed between the nose extension 23 and discharge port 17, the contact member 24 being provided so as to project from free end of the nose extension.