In the present invention, a high-voltage side electrode constituent part includes a dielectric electrode and metal electrodes formed on the upper surface of the dielectric electrode. The dielectric electrode has a structure in which a film thickness is continuously changed along an X direction. That is, the film thickness of the right end of the dielectric electrode is set to a thickness dA1; and the film thickness of the left end is set to a thickness dB1 (>dA1), and is continuously increased from the right end to the left end along the X direction.