As shown in FIG. 4, the groove 22 is provided with plurality of gas introduction paths 24 that are configured to be through holes at the first end thereof, and arranged with equal intervals in the width direction. The number of gas introduction paths 24 can be any number; however, it is preferably at least two as shown in the present embodiment. The configuration including the at least two gas introduction paths 24 readily make the gas flow in the gas channel 25 more laminar flow because the gas is introduced into the gas channel 25 from at least two locations. The plurality of gas introduction paths 24 are preferably arranged in the width direction so that gas is widely introduced at the location where the gas is introduced into the gas channel 25. The gas introduction path 24 may be a single wide open hole in the width direction.

As shown in FIGS. 2 and 3, the low-voltage electrode 20 and a gas buffer substrate 26 are stacked such that the lower surface of the low-voltage electrode 20 (the surface being opposite to a surface facing the dielectric substrate 12) is in contact with the gas buffer substrate 26. The gas buffer substrate 26 has a cavity 27 thereinside.

The cavity 27 inside the gas buffer substrate 26 is connected to a gas delivery device 40 (see FIG. 2). The gas that has been delivered from the gas delivery device 40 is stored in the cavity 27 and then flows through the plurality of gas introduction paths 24 into the gas channels 25.