The dielectric substrate 12 can be made of any materials; however it is preferably made of a material with the lowest permittivity when possible. Ceramics are preferred from a standpoint of durability. Examples of the ceramics include alumina, aluminum nitride, and steatite. Alumina, aluminum nitride, and steatite have comparatively low relative permittivity, comparatively high strength, and excellent durability. Hence, the dielectric substrate 12 being made of alumina, aluminum nitride, or steatite, enables the amount of plasma generated per unit power to be increased. In addition, this configuration reduces a risk of damage even when the dielectric substrate 12 is made thinner. Furthermore, the dielectric substrate 12 may be made of a material that contains a substance that assists electron generation in addition to the above dielectric materials, which are its base material. Examples of the substance that assists electron generation include silver, platinum, copper, carbon, and transition metal compounds. Applying an electric field to the substance that assists electron generation causes initial electrons to be generated and released into a discharge space. This favorably benefits the starting performance. The dielectric substrate 12 may preferably contain the substance, which assists the electron generation, of 1% by weight or less with respect to the dielectric substrate 12 as a whole (when the dielectric substrate 12 is set to 100% by weight). When the substance that assists electron generation is made to be contained in the dielectric substrate 12, the reason for setting the content of the substance that assists electron generation to be 1% by weight or less contributes to suppressing the present substance from evaporating and dispersing due to the discharge as much as possible. The material that evaporates and disperses with the discharge is blown out with the plasma as fine particles and contaminates the object to be irradiated. When the substance that assists electron generation is made to be contained in the dielectric substrate 12, the content of the substance is preferably 0.05% by weight or more.