A apparatus comprising a flow tube with a gas inlet at one end and a gas outlet at another end, a plasma reactor arranged in the flow tube, and a pulse generating power source, the plasma reactor comprising a plurality of electrode panels (21, 22) each including a dielectric body and an electrode (24, 25, 51, 53, 61, 63, 70, 80) included therein, the electrode panels (21, 22) having each a rectangular plate-like form and being disposed alternately in parallel at regular intervals, the electrode panels (21, 22) extending in a direction from the gas inlet toward the gas outlet, the plasma reactor being adapted to generate plasma by dielectric barrier discharge between the electrode panels (21, 22), wherein: the electrode panels (21, 22) is obtainable through firing of a ceramic green sheet, an electrically conductive paste on the surface of the ceramic green sheet, and an optional further ceramic green sheet on the ceramic green sheet, each of the electrodes (24, 25, 51, 53, 61, 63, 70, 80) has a plurality of through holes (33, 43, 52, 54, 62, 64, 75, 76) each penetrating the electrode (24, 25, 51, 53, 61, 63, 70, 80) in a thickness direction and having a shape having a corner (34, 44) in plan view; and at least some of the corners of the through holes (33, 52, 62, 75, 76) of a first electrode (24, 51, 61, 70, 80) of a first electrode panel of the electrode panels (21, 22) do not overlap with at least some of the corners of the through holes (43, 54, 64, 75, 76) of a second electrode (25, 53, 63, 70, 80) of a second electrode panel of the electrode panels (21, 22) adjacent to the first electrode (24, 51, 61, 70, 80) so that, during operation of the plasma reactor, the dielectric barrier discharge occurs between opening edges of the through holes (33, 52, 62, 75, 76) of the first electrode (24, 51, 61, 70, 80) of the first electrode panel and opening edges of the through holes (43, 54, 64, 75, 76) of the second electrode (25, 53, 63, 70, 80) of the second electrode panel, which opening edges are located obliquely downward of the opening edges of the through holes (33, 52, 62, 75, 76) of the first electrode (24, 51, 61, 70, 80) of the first electrode panel. the electrode panels (21, 22) is obtainable through firing of a ceramic green sheet, an electrically conductive paste on the surface of the ceramic green sheet, and an optional further ceramic green sheet on the ceramic green sheet, each of the electrodes (24, 25, 51, 53, 61, 63, 70, 80) has a plurality of through holes (33, 43, 52, 54, 62, 64, 75, 76) each penetrating the electrode (24, 25, 51, 53, 61, 63, 70, 80) in a thickness direction and having a shape having a corner (34, 44) in plan view; and at least some of the corners of the through holes (33, 52, 62, 75, 76) of a first electrode (24, 51, 61, 70, 80) of a first electrode panel of the electrode panels (21, 22) do not overlap with at least some of the corners of the through holes (43, 54, 64, 75, 76) of a second electrode (25, 53, 63, 70, 80) of a second electrode panel of the electrode panels (21, 22) adjacent to the first electrode (24, 51, 61, 70, 80) so that, during operation of the plasma reactor, the dielectric barrier discharge occurs between opening edges of the through holes (33, 52, 62, 75, 76) of the first electrode (24, 51, 61, 70, 80) of the first electrode panel and opening edges of the through holes (43, 54, 64, 75, 76) of the second electrode (25, 53, 63, 70, 80) of the second electrode panel, which opening edges are located obliquely downward of the opening edges of the through holes (33, 52, 62, 75, 76) of the first electrode (24, 51, 61, 70, 80) of the first electrode panel.