It is an high efficiency and low cost apparatus for simultaneous dry desulfurization and denitration (10), capable of simultaneous oxidation of nitrogen monoxide and sulfur dioxide by chain reaction with OH radical, provided with an OH radical supplying unit (12), a reactor (14), a sulfuric acid recovering unit (16), and a nitric acid recovering unit (18). Exhaust gas at 600-800.degree. C. containing sulfur compounds from a boiler (2) is introduced into the reactor (14), nitric acid is spray-supplied from an OH radical supplying unit (12) into the reactor (14), sulfur dioxide and nitrogen monoxide are simultaneously oxidized with OH radicals generated from pyrolysis of nitric acid as an initiator to form sulfur trioxide and nitrogen dioxide, thereby exhaust gas is treated.