Since the discharged gas flowing near the outlet passage contains Cu(I)hfacTMVS scarcely, the gap 73 is opened to the labyrinthine passage 62 to facilitate handling the trap body 56. [0091] The trap body 56 or the trapping plates 60 may be formed of a material other than copper, such as a stainless steel, and the trapping surfaces of the trap body 56 or the trapping plates 60 may not be copper-pla