Further, the carbon fibers 8 and the electrodes 11 and 12 (the electrode 12 is in conduction with the carbon fibers 8 and faces against the electrode 11 with the carbon fibers 8 therebetween in the case 7) have a noble metal such as palladium (Pd), platinum (Pt), iridium (Ir) or tantalum (Ta) coated at least on their surfaces.