PURPOSE: To prevent the swelling of plastic parts by forming a lipophobic film on a plastic needle cap mounted at the rear end of a needle to keep a lubricating oil from being absorbed by the needle cap.

CONSTITUTION: A hydrophobic/lipophobic film 37 made of a fluorine based polymer or the like is formed at the end of a cap guide 26, a plastic needle cap 32 made of nylon or the like and the tip of an armature 33. An electromagnet 29 is excited and attracts the armature 33 to have a needle 27 flying and hitting a platen for printing. This brings the lubricating oil impregnated into a felt 28 and a lubricating oil fed to a joint between the armature 33 and a plate spring 35 close to the needle cap 32 and the cap guide 26. But the lipophobic film 37 prevents the swelling of the parts 32 and 26 thereby eliminating friction of the needle cap 32.