PURPOSE: To stably perform the efficient treatment of a waste substance at a high removal rate or a high decomposition rate.

CONSTITUTION: The interior of a treatment container 21 is partitioned into a discharge chamber 25 having the dielectric plate 23 closely bonded to a high voltage electrode 22 exposed on the inner surface side thereof and a decomposition treatment chamber 26 by a window 24 and a discharge electrode 27 having a large number of openings is provided in opposed relation to the dielectric plate 23 in the discharge chamber 25 and a treatment gas introducing pipe 30 equipped with a heater 33 and a treatment gas discharge pipe 34 are attached to the decomposition treatment chamber 26. The combustion gas containing NOx and SOx thermally excited by the heater 33 is introduced into the decomposition treatment chamber 26 and irradiated with vacuum UV rays generated accompanied by dielectric barrier wall discharge to be optically dissociated to produce a large amt. of free radicals and NOx and SOx are decomposed by the free radicals to be made harmless.