including a plurality of projecting electrode is uniform being greater than the gap between the second discharge electrode part 302 accordingly it is possible to easily perform a discharge in the meantime as described above the gap g 2 between the first discharge electrode part 300 may be greater 1 to 5 times further 1 5 to 3 times than the gaps g 1 g 3 between the second discharge electrode part 302 as described above the present invention is effective just when the gap g 2 between the first discharge electrode part 300 is greater 1 times than gaps g 1 g 3 between the second discharge electrode part 302 moreover there is a problem that the discharge voltage between y and z drastically rises if the gap g 2 between the first discharge electrode part 300 is less 5 times than the gaps g 1 g 3 between the second discharge electrode part 302 preferably the gap g 2 between the first discharge electrode part 300 may be greater 1 5 to 3 times than the gaps g 1 g 3 between the second discharge electrode part 302 only when the gap g 2 between the first discharge electrode part 300 is greater over 1 5 times than gaps g 1 g 3 between the second discharge electrode part then the discharge between the first discharge electrode part and the second discharge electrode part is uniformly generated additionally there is a problem that if the gap g 2 between the first discharge electrode part 300.