PROBLEM TO BE SOLVED: To enhance contamination resistance and reduce spark discharge requiring voltage by forming a semi-surface spark discharge gap between the base part of the tip part of a central electrode and at least one earthed electrode of a plurality of earthed electrodes, and forming an aerial spark discharge gap between the end part of the tip part of the central electrode and other electrodes. ;SOLUTION: An aerial spark discharge gap G1 is formed between facing earthed electrodes 41, 43 of a plurality of earthed electrodes and the side surface of the end part 32 of the tip part 31 of a central electrode 3. The remaining facing earthed electrodes 42, 44 are positioned on the side of the front end part 21 of an insulator 2, and a semi-surface spark discharge gap G2 comprising the surface along the front end part 21 of the insulator 2 and a gap G3 between the front end surface 21 and the tip surface is formed between the base part 33 of the tip part 31 of the central electrode 3 and the earthed electrodes 42, 44. When a conductive deposit such as carbon is accumulated on the insulator 2, surface electric resistance is decreased. When discharge voltage in the gap G2 is made smaller than the discharge voltage of the gap G1, spark is produced in the gap G2, and carbon is burned and removed.;COPYRIGHT: (C)1998,JPO
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